[GTK] Fix NPE on POST requests when response has no Content-Type

The NPE was output to System.err because nothing catches it, but
its running on a thread. The code worked, but System.err would have

```
Exception in thread "Thread-2" java.lang.NullPointerException: Cannot invoke "String.indexOf(int)" because "content_type" is null
	at org.eclipse.swt.browser.WebKit.lambda$14(WebKit.java:2186)
	at java.base/java.lang.Thread.run(Thread.java:1583)
```

or similar.

Author: jonahgraham

bundles/org.eclipse.swt/Eclipse SWT WebKit/gtk/org/eclipse/swt/browser/WebKit.java

@@ -2200,19 +2200,21 @@ public boolean setUrl (String url, String postData, String[] headers) {
 					{ // Extract result meta data
 						// Get Media Type from Content-Type
 						String content_type = conn.getContentType();
-						int parameterSeparatorIndex = content_type.indexOf(';');
-						mime_type = parameterSeparatorIndex > 0 ? content_type.substring(0, parameterSeparatorIndex) : content_type;
-
-						// Get Encoding if defined
-						if (content_type.indexOf(';') > 0) {
-							String [] attrs = content_type.split(";");
-							for (String attr : attrs) {
-								int i = attr.indexOf('=');
-								if (i > 0) {
-									String key = attr.substring(0, i).trim();
-									String value = attr.substring(i + 1).trim();
-									if ("charset".equalsIgnoreCase(key)) { //$NON-NLS-1$
-										encoding_type = value;
+						if (content_type != null) {
+							int parameterSeparatorIndex = content_type.indexOf(';');
+							mime_type = parameterSeparatorIndex > 0 ? content_type.substring(0, parameterSeparatorIndex) : content_type;
+
+							// Get Encoding if defined
+							if (content_type.indexOf(';') > 0) {
+								String [] attrs = content_type.split(";");
+								for (String attr : attrs) {
+									int i = attr.indexOf('=');
+									if (i > 0) {
+										String key = attr.substring(0, i).trim();
+										String value = attr.substring(i + 1).trim();
+										if ("charset".equalsIgnoreCase(key)) { //$NON-NLS-1$
+											encoding_type = value;
+										}
 									}
 								}
 							}

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_browser_Browser.java

@@ -1284,6 +1284,43 @@ public void test_setUrl_remote_with_post() throws IOException {
 	}
 }
 
+@Test
+public void test_setUrl_remote_with_post_no_content_type() throws IOException {
+	try (var server = new EchoHttpServer() {
+		@Override
+		protected void setResponseHeaders(HttpExchange exchange) {
+			// omit calling super so that no content type is set
+		}
+	}) {
+		try (var capture = new CapturedOutput()) {
+			String url = server.postEchoUrl();
+			String postData = "test_setUrl_remote_with_post";
+
+			Runnable browserSetFunc = () -> {
+				testLog.append("Setting Browser url to:" + url);
+				boolean opSuccess = browser.setUrl(url, postData, null);
+				assertTrue(opSuccess);
+			};
+
+			final AtomicReference<Boolean> completed = new AtomicReference<>(false);
+			browser.addProgressListener(completedAdapter(event -> {
+				testLog.append("ProgressListener fired");
+				completed.set(true);
+			}));
+			browserSetFunc.run();
+			shell.open();
+
+			boolean hasFinished = waitForPassCondition(() -> completed.get().booleanValue());
+			assertTrue(hasFinished);
+
+			String lowerCase = browser.getText().toLowerCase();
+			assertTrue(lowerCase.contains("<title>test_setUrl_remote_with_post</title>".toLowerCase()), "Browser getText was: " + browser.getText());
+			assertTrue(lowerCase.contains("</html>"), "Browser getText was: " + browser.getText());
+			capture.assertNoOutput();
+		}
+	}
+}
+
 @Test
 public void test_setUrl_post_invalid_url() {
 	assumeTrue(SwtTestUtil.isLinux, "Handling of invalid URLs is platform dependent");