Handle empty password from broader set of MySQL clients

Some MySQL clients (e.g. libmysql) send a single null byte to indicate an empty password, while others (e.g. mariadb) send an empty packet. This matches MySQL server's own handling:

```c
if (!pkt_len || (pkt_len == 1 && *pkt == 0))
```

(Source: https://github.com/mysql/mysql-server/blob/8.0/sql/auth/sha2_password.cc)

Author: ramnes

server/auth.go

@@ -18,6 +18,15 @@ var (
 	ErrAccessDeniedNoPassword = fmt.Errorf("%w without password", ErrAccessDenied)
 )
 
+// isEmptyPassword returns true if the auth data represents an empty password.
+// Some clients send an empty packet (len == 0), while others (e.g. MySQL's libmysql)
+// send a single null byte. This matches MySQL server's own handling:
+// if (!pkt_len || (pkt_len == 1 && *pkt == 0))
+// See: https://github.com/mysql/mysql-server/blob/8.0/sql/auth/sha2_password.cc
+func isEmptyPassword(authData []byte) bool {
+	return len(authData) == 0 || (len(authData) == 1 && authData[0] == 0)
+}
+
 func (c *Conn) compareAuthData(authPluginName string, clientAuthData []byte) error {
 	if authPluginName != c.credential.AuthPluginName {
 		err := c.writeAuthSwitchRequest(c.credential.AuthPluginName)
@@ -66,7 +75,7 @@ func scrambleValidation(cached, nonce, scramble []byte) bool {
 }
 
 func (c *Conn) compareNativePasswordAuthData(clientAuthData []byte, credential Credential) error {
-	if len(clientAuthData) == 0 {
+	if isEmptyPassword(clientAuthData) {
 		if credential.Password == "" {
 			return nil
 		}
@@ -84,8 +93,7 @@ func (c *Conn) compareNativePasswordAuthData(clientAuthData []byte, credential C
 }
 
 func (c *Conn) compareSha256PasswordAuthData(clientAuthData []byte, credential Credential) error {
-	// Empty passwords are not hashed, but sent as empty string
-	if len(clientAuthData) == 0 {
+	if isEmptyPassword(clientAuthData) {
 		if credential.Password == "" {
 			return nil
 		}
@@ -123,8 +131,7 @@ func (c *Conn) compareSha256PasswordAuthData(clientAuthData []byte, credential C
 }
 
 func (c *Conn) compareCacheSha2PasswordAuthData(clientAuthData []byte) error {
-	// Empty passwords are not hashed, but sent as empty string
-	if len(clientAuthData) == 0 {
+	if isEmptyPassword(clientAuthData) {
 		if c.credential.Password == "" {
 			return nil
 		}

server/auth_switch_response.go

@@ -71,7 +71,7 @@ func (c *Conn) handleCachingSha2PasswordFullAuth(authData []byte) error {
 }
 
 func (c *Conn) checkSha2CacheCredentials(clientAuthData []byte, credential Credential) error {
-	if len(clientAuthData) == 0 {
+	if isEmptyPassword(clientAuthData) {
 		if credential.Password == "" {
 			return nil
 		}

server/auth_switch_response_test.go

@@ -25,6 +25,18 @@ func TestCheckSha2CacheCredentials_EmptyPassword(t *testing.T) {
 			serverPassword: "secret",
 			wantErr:        ErrAccessDeniedNoPassword,
 		},
+		{
+			name:           "null byte client auth, empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "",
+			wantErr:        nil,
+		},
+		{
+			name:           "null byte client auth, non-empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "secret",
+			wantErr:        ErrAccessDeniedNoPassword,
+		},
 	}
 
 	for _, tt := range tests {

server/auth_test.go

@@ -32,6 +32,18 @@ func TestCompareNativePasswordAuthData_EmptyPassword(t *testing.T) {
 			serverPassword: "secret",
 			wantErr:        ErrAccessDeniedNoPassword,
 		},
+		{
+			name:           "null byte client auth, empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "",
+			wantErr:        nil,
+		},
+		{
+			name:           "null byte client auth, non-empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "secret",
+			wantErr:        ErrAccessDeniedNoPassword,
+		},
 	}
 
 	for _, tt := range tests {
@@ -68,6 +80,18 @@ func TestCompareSha256PasswordAuthData_EmptyPassword(t *testing.T) {
 			serverPassword: "secret",
 			wantErr:        ErrAccessDeniedNoPassword,
 		},
+		{
+			name:           "null byte client auth, empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "",
+			wantErr:        nil,
+		},
+		{
+			name:           "null byte client auth, non-empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "secret",
+			wantErr:        ErrAccessDeniedNoPassword,
+		},
 	}
 
 	for _, tt := range tests {
@@ -104,6 +128,18 @@ func TestCompareCacheSha2PasswordAuthData_EmptyPassword(t *testing.T) {
 			serverPassword: "secret",
 			wantErr:        ErrAccessDeniedNoPassword,
 		},
+		{
+			name:           "null byte client auth, empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "",
+			wantErr:        nil,
+		},
+		{
+			name:           "null byte client auth, non-empty server password",
+			clientAuthData: []byte{0x00},
+			serverPassword: "secret",
+			wantErr:        ErrAccessDeniedNoPassword,
+		},
 	}
 
 	for _, tt := range tests {