Co-authored-by: Mike Hunhoff <mike.hunhoff@gmail.com>

To make sure that Ghidrathon plugin aligns with configure_ghidrathon.py
we need to make sure that the GHIDRATHON_SAVE_PATH is never set to "".

The following commit fixes incorrect login in the previous commit.
Please refer to the explination below:

if GHIDRATHON_SAVE_PATH is set to any value (other than None or ""),
check if it exists and is a directory.

elif GHIDRATHON_SAVE_PATH is an empty string, report an invalid
directory to avoid confusion.

else GHIDRATHON_SAVE_PATH is not, use default install path.

Added log messages to indicate whether the file path was read from the
environment variable or the default path was used.