Fix FormatException cause by empty json file

[bothyculture]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Fixes an exception caused by empty json files

:arrow_heading_down: What is the current behavior?

Throws FormatException: Unexpected end of input (at character 1)

:new: What is the new behavior (if this is a feature change)?

No error logged

:boom: Does this PR introduce a breaking change?

No

:bug: Recommendations for testing

Added a test for this

:memo: Links to relevant issues/docs

https://github.com/Baseflow/flutter_cache_manager/issues/450

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines (code style guide)
• [x] Relevant documentation was updated
• [x] Rebased onto current develop