dist/tools/compile_commands: fix error handling

detect_includes_and_version_gcc() previously only detected the includes,
but has been extended to also return the version. This is done by
returning a tuple, with the first item being the list of include paths,
and the second being the version. In the error handling the script still
returns only an empty list of includes, but not an empty version. This
fixes the issue.

dist/tools/compile_commands/compile_commands.py

@@ -35,7 +35,7 @@ def detect_includes_and_version_gcc(compiler):
     except FileNotFoundError:
         msg = f"Compiler {compiler} not found, not adding system include paths\n"
         sys.stderr.write(msg)
-        return []
+        return ([], "")
 
     stderrdata = stderrdata.decode("utf-8")
     version = REGEX_VERSION.search(stderrdata).group(1)