Extend existence check in URLImageDescriptor when running without OSGi

This extends the check added in cff785d
to also consider the case when OSGi isn't running. Otherwise a path to a
non-existent file is returned, thus breaking the contract specified in
the JavaDoc.

Author: ptziegler

bundles/org.eclipse.jface/src/org/eclipse/jface/resource/URLImageDescriptor.java

@@ -249,8 +249,12 @@ private static String getxPath(String name, int zoom) {
 	private static String getFilePath(URL url, boolean logIOException) {
 		try {
 			if (!InternalPolicy.OSGI_AVAILABLE) {
-				if (FILE_PROTOCOL.equalsIgnoreCase(url.getProtocol()))
-					return IPath.fromOSString(url.getFile()).toOSString();
+				if (FILE_PROTOCOL.equalsIgnoreCase(url.getProtocol())) {
+					String filePath = IPath.fromOSString(url.getFile()).toOSString();
+					if (Files.exists(Path.of(filePath))) {
+						return filePath;
+					}
+				}
 				return null;
 			}
 			url = resolvePathVariables(url);