fix raw string indentation with cstyle + C++

The presence or absence of a delimiter did not have the same behavior:

• without a delimiter, the result was an invalid regex ()" -> Unmatched ')') and there was never any indentation
• with a delimiter, either the line was not empty and there was no indentation, or the line was empty and the indenter considered that it was not in a raw-string (the raw-string was considered C code).

The new version indents at the same level as the previous non-empty line of the raw-string (no indentation after the first line).