Allows empty lines in RPP files

• previously, the parse failed if it encountered an empty line due to an IndexError
• some extensions apparently can add empty lines
• an example of such a file is in tests/data/with_empty_lines.RPP

Hello @qubitstream, I'm sorry for the late notice.

Does the extension work OK if you remove the empty line and load the project in REAPER?

The way it's implemented in this PR makes it unable to restore those empty lines when writing the file.

Instead, starts_with_quote and starts_with_pipe (scanner.py:68-75) should just return False if the string is empty, similarly to how the startswith method work in python.