Fix external tools package documentation missing

See #1248

Author: Philippe Gil

source/gpr/lsp-gpr_external_tools.adb

@@ -203,7 +203,7 @@ package body LSP.GPR_External_Tools is
       if not Exists (Name) then
          Add (Name, Projects);
       end if;
-      if Get_Package_Description (Name)'Length > 0
+      if Get_Package_Description (Name)'Length = 0
         and then Description'Length > 0
       then
          Set_Package_Description (Name, Description);

testsuite/gpr_lsp/completion_package_resolve/test.json

@@ -578,6 +578,34 @@
         }
       ]
     }
+  },
+   {
+    "send": {
+      "request": {
+        "id": 13,
+        "method": "completionItem/resolve",
+        "params": {
+          "label": "Ide",
+          "insertTextFormat": 1,
+          "data": [
+            "Ide"
+          ]
+        }
+      },
+      "wait": [
+        {
+          "id": 13,
+          "result": {
+            "label": "Ide",
+            "documentation": "This package specifies the options used by 'gnatstudio' IDE.",
+            "insertTextFormat": 1,
+            "data": [
+              "Ide"
+            ]
+          }
+        }
+      ]
+    }
   },
   {
     "stop": {