Fix: Default to empty string as content type

[localheinz]

This PR

• defaults to an empty string as content type when ChainUriRetriever::retrieve() wasn't invoked yet, dropping the nullable return type declaration

[codecov]

Codecov Report

Merging #132 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #132   +/-   ##
=======================================
  Coverage       100%   100%           
+ Complexity      129    128    -1     
=======================================
  Files            30     30           
  Lines           404    404           
=======================================
  Hits            404    404

Impacted Files	Coverage Δ	Complexity Δ
...rc/JsonSchema/Uri/Retrievers/ChainUriRetriever.php	100% <100%> (ø)	6 <1> (-1)	⬇️

---

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 fee48f5...b9ffb51. Read the comment docs.