Fix wrong assertion in assertNotEmpty

The non-null assertion was made on the exception message instead of the string
to check, causing a NullPointerException if the string to check was null.

extractor/src/test/java/org/schabi/newpipe/extractor/ExtractorAsserts.java

@@ -56,7 +56,7 @@ public class ExtractorAsserts {
     }
 
     public static void assertNotEmpty(@Nullable String message, String stringToCheck) {
-        assertNotNull(message, stringToCheck);
+        assertNotNull(stringToCheck, message);
         assertFalse(stringToCheck.isEmpty(), message);
     }