Return Platform.OS_UNKNOWN in case of null

In some cases the return value of Platform.getOS() can be return null
but its API doc states it return Platform.OS_UNKNOWN is the OS can not
be found.

Fix eclipse-platform/eclipse.platform.ui#3604

Author: laeubi

runtime/bundles/org.eclipse.core.runtime/src/org/eclipse/core/internal/runtime/InternalPlatform.java

@@ -471,7 +471,11 @@ public String getOption(String option) {
 	}
 
 	public String getOS() {
-		return getContextProperty(PROP_OS);
+		String property = getContextProperty(PROP_OS);
+		if (property == null) {
+			return Platform.OS_UNKNOWN;
+		}
+		return property;
 	}
 
 	public String getWS() {

runtime/bundles/org.eclipse.core.runtime/src/org/eclipse/core/runtime/Platform.java

@@ -22,6 +22,7 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.MissingResourceException;
+import java.util.Objects;
 import java.util.ResourceBundle;
 import org.eclipse.core.internal.runtime.AuthorizationHandler;
 import org.eclipse.core.internal.runtime.InternalPlatform;
@@ -79,7 +80,7 @@ private OS() {
 		 * @since 3.30
 		 */
 		public static boolean is(String osString) {
-			return Platform.getOS().equals(osString);
+			return Objects.equals(Platform.getOS(), osString);
 		}
 
 		/**