[feat] Accept non-existing file paths in report-converter

[bruntib]

Some analyzer tools (e.g. sanitizers) give an output that contains relative source file paths. When report-converter is executed from a directory where these absolute paths are not valid, then reports are simply not generated. If all reports contain such a relative path, then this message is given by report-converter:

No 'lsan' results can be found in '...'

This message is obviously false.

This patch allows non-existing file paths in the analyzer outputs. Using further CodeChecker commands (store, parse, etc.) will emit an error message on these paths anyway, but at least we get rid of the incomprehensible error message above.

Fixes #4280