Handle is_file error gracefully

[cabhishek]

from genie.jobs import PrestoJob
rj = PrestoJob().script('select * from dual\x00').execute()

For python2 above code throws
TypeError: stat() argument 1 must be encoded string without null bytes, not unicode

and for python3 it throws

ValueError: embedded null byte

The error actually occurs during os.path.isfile check here. This PR handles invalid path and returns False which should then surface the actual error (if any) in the script itself e.g Parsing error for Presto Engine.

Additional context -

• Python bug - https://bugs.python.org/issue33721
• Cpython PR - bpo-33721: Make some os.path functions and pathlib.Path methods be tolerant to invalid paths.  python/cpython#7695