Deal with empty strings as if they're null in the format

source/net/filebot/format/ExpressionFormatFunctions.java

@@ -31,6 +31,9 @@ public class ExpressionFormatFunctions {
 				return null;
 			}
 		}
+		if (object instanceof CharSequence && object.toString().isEmpty()) {
+			return null;
+		}
 		return object;
 	}