Preprocess is not functioning in certain conditions

[sarthakpati]

Describe the bug
When 'path' is absent from the subject dictionary in the first image of a subject, the entire process fails because subject["1"]["path"][0] is empty.

To Reproduce
Steps to reproduce the behavior:

1. Put a case where subject["1"]["path"][0] is empty.
2. Run gandlf_preprocess on it using any config.
3. See error

Expected behavior
Even if the 'path' is absent, it should get picked up from the 'path_to_metadata' key.

Screenshots
N.A.

GaNDLF Version

0.0.16-dev

Desktop (please complete the following information):
N.A.

Additional context
N.A.