Handle loading invalid JSON from db

[rpkilby]

This handles loading invalid JSON values (such as an empty string) from the database. While the field should never save any invalid JSON, it's possible that an invalid value was loaded into the DB through other means. A RuntimeWarning is issued, however the value is still loaded as a string.

Fixes #199 and fixes #100.

[codecov]

Codecov Report

Merging #248 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #248   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines         103    109    +6     
  Branches       14     14           
=====================================
+ Hits          103    109    +6

Impacted Files	Coverage Δ
src/jsonfield/fields.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5e0de07...fa538c2. Read the comment docs.