Fix PatternValidator & Add PatternValidatorTest

src/main/java/at/pcgamingfreaks/mkvaudiosubtitlechanger/config/validator/PatternValidator.java

@@ -6,6 +6,8 @@ import java.util.regex.Pattern;
 import java.util.regex.PatternSyntaxException;
 
 public class PatternValidator extends ConfigValidator<Pattern> {
+    private static final Pattern EMPTY_PATTERN = Pattern.compile("");
+
     public PatternValidator(ConfigProperty property, boolean required, Pattern defaultValue) {
         super(property, required, defaultValue);
     }
@@ -15,12 +17,12 @@ public class PatternValidator extends ConfigValidator<Pattern> {
         try {
             return Pattern.compile(value);
         } catch (PatternSyntaxException e) {
-            return null;
+            return EMPTY_PATTERN;
         }
     }
 
     @Override
     boolean isValid(Pattern result) {
-        return result != null;
+        return !result.equals(EMPTY_PATTERN);
     }
 }