fix(workflow-server): CharacterReplacementStrategy Empty Result Handling

While artificial if used "as intended", the replacement strategy should
be able not to provide unexpected results for unexpected successful
empty matches, like for `.*`.

apps/workflow-server/gcc-workflow-server-facade/gcc-restclient-facade/src/main/java/com/coremedia/labs/translation/gcc/facade/config/CharacterReplacementStrategy.java

@@ -28,16 +28,20 @@ public enum CharacterReplacementStrategy {
   /**
    * Replace with an underscore.
    */
-  UNDERSCORE(mr -> "_"),
+  UNDERSCORE(mr -> replaceIfNotEmpty(mr, "_")),
   /**
    * Replace with a question mark.
    */
-  QUESTION_MARK(mr -> "?"),
+  QUESTION_MARK(mr -> replaceIfNotEmpty(mr, "?")),
   /**
    * Replace with a Unicode code point.
    */
   UNICODE_CODE_POINT(mr -> {
-    int codePoint = mr.group().codePointAt(0);
+    String group = mr.group();
+    if (mr.group().isEmpty()) {
+      return "";
+    }
+    int codePoint = group.codePointAt(0);
     return String.format("U+%04X", codePoint);
   }),
   ;
@@ -103,6 +107,11 @@ public static Optional<CharacterReplacementStrategy> fromString(@Nullable String
     return Optional.empty();
   }
 
+  @NonNull
+  private static String replaceIfNotEmpty(@NonNull MatchResult mr, @NonNull String replacement) {
+    return mr.group().isEmpty() ? "" : replacement;
+  }
+
   /**
    * Used to eventually support camel-case, snake-case, and kebab-case.
    *