Allows empty lines in RPP files

[qubitstream]

• 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

[Perlence]

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?

[kokounet]

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.