Print a clear warning when indentation sample is missing

Currently KDevelop does some useless work, then prints an unrelated and
confusing warning: "indent returned empty text for style ..."

Steps to make KDevelop print this warning:
1. Check "Override Kate Indentation Mode" and/or "Add Kate Modeline"
   checkbox in Source Formatter configuration.
2. Configure any Custom Script Formatter style for a language, e.g. C++.
3. Open a document in the configured language, e.g. a .cpp file.
4. Ensure that no text is selected, then trigger Reformat Source action.

Note that !273
implements ClangSupport::indentationSample(). But other language plugins
don't override this function. Once !273 is merged, this message will
still be printed for Python documents.