[SYNPY-1453] Guard around modified time item in cache

[BryanFauble]

Problem:

1. In the cache if a cache value was a dictionary but it did not contain the exact key of modified_time the code would return back the parent dict - causing further code to fail that was expecting a string to be returned.

Solution:

1. Adding a guard statement in _get_cache_modified_time that verified that the data returned is going to be a string or the value for modified_time

Testing:

Added unit tests around the logic that verify the behavior if the cache map:

1. Is a dict but does not contain modified_time key
2. Is a dict but contains neither of the expected keys
3. The entry is an empty dict
4. The entry is None

[sonarcloud[bot]]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud