TINKERPOP-3225 conjoin has incorrect null handling

[GumpacG]

https://issues.apache.org/jira/browse/TINKERPOP-3225

// before
gremlin> g.inject([null]).conjoin("-")
==>null
gremlin> g.inject([null, null]).conjoin("-")
==>null

// after
gremlin> g.inject([null]).conjoin("+")
==>
gremlin> g.inject([null, null]).conjoin("+")
==>

Added tests to core and feature tests.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.50%. Comparing base (9b46b67) to head (fb7dfdf).
⚠️ Report is 445 commits behind head on 3.7-dev.

Additional details and impacted files

@@              Coverage Diff              @@
##             3.7-dev    #3372      +/-   ##
=============================================
- Coverage      76.14%   75.50%   -0.65%     
- Complexity     13152    13164      +12     
=============================================
  Files           1084     1092       +8     
  Lines          65160    67191    +2031     
  Branches        7285     7391     +106     
=============================================
+ Hits           49616    50731    +1115     
- Misses         12839    13839    +1000     
+ Partials        2705     2621      -84     

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