[BUG] Empty .jwks file causes sign-in errors

[aristizabal95]

Describe the bug
In some strange edge cases, the .jwks file that resides in ~/.medperf_config/.tokens can end up empty. When that happens, Authentication fails, as medperf tries to read this file as a JSON object and can't figure out how to parse empty files.

To Reproduce

1. Create an empty .jwks file in ~/.medperf_config/.tokens. Rename the previos one if needed
2. Sign out of medperf.
3. Try to sign back in. A JSONDecodeError will be raised.

Expected behavior
While the expected behavior should be to completely avoid generating empty .jwks file, a good additional behavior is to try to read the file and if it fails remove the corrupted one an regenerate it.