Allow credentials to be nullable in HttpJsonCallOptions

[andreamlin]

This allows generated unit tests, which don't have credentials set, to still be able to create the tested client object.

[codecov-io]

Codecov Report

Merging #518 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #518   +/-   ##
=========================================
  Coverage     72.64%   72.64%           
  Complexity      853      853           
=========================================
  Files           169      169           
  Lines          3864     3864           
  Branches        294      294           
=========================================
  Hits           2807     2807           
  Misses          914      914           
  Partials        143      143

Impacted Files	Coverage Δ	Complexity Δ
...m/google/api/gax/httpjson/HttpJsonCallOptions.java	50% <ø> (ø)	1 <0> (ø)	⬇️

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[andreamlin]

PTAL

[vam-google]

LGTM