bugfix: saga ExpressionEvaluator not support null value

[long187]

Ⅰ. Describe what this PR did

Fix Saga ExpressionEvaluator not support null value

Ⅱ. Does this pull request fix one issue?

#1829

[codecov-io]

Codecov Report

Merging #1837 into develop will increase coverage by 0.1%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             develop    #1837     +/-   ##
============================================
+ Coverage      54.95%   55.06%   +0.1%     
  Complexity      2352     2352             
============================================
  Files            421      421             
  Lines          14266    14266             
  Branches        1682     1711     +29     
============================================
+ Hits            7840     7855     +15     
  Misses          5681     5681             
+ Partials         745      730     -15

Impacted Files	Coverage Δ	Complexity Δ
...rotobuf/convertor/RegisterTMResponseConvertor.java	84.84% <0%> (+3.03%)	3% <0%> (ø)	⬇️
...rotobuf/convertor/RegisterRMResponseConvertor.java	84.84% <0%> (+3.03%)	3% <0%> (ø)	⬇️
...otobuf/convertor/GlobalBeginResponseConvertor.java	96.96% <0%> (+3.03%)	3% <0%> (ø)	⬇️
...obuf/convertor/BranchRollbackRequestConvertor.java	96.42% <0%> (+3.57%)	3% <0%> (ø)	⬇️
...otobuf/convertor/BranchCommitRequestConvertor.java	100% <0%> (+3.57%)	3% <0%> (ø)	⬇️
...otobuf/convertor/BranchReportRequestConvertor.java	96.29% <0%> (+3.7%)	3% <0%> (ø)	⬇️
...protobuf/convertor/RegisterRMRequestConvertor.java	95.83% <0%> (+4.16%)	3% <0%> (ø)	⬇️
...protobuf/convertor/RegisterTMRequestConvertor.java	100% <0%> (+4.54%)	3% <0%> (ø)	⬇️
...otobuf/convertor/GlobalStatusRequestConvertor.java	100% <0%> (+5.26%)	3% <0%> (ø)	⬇️
...obuf/convertor/GlobalRollbackRequestConvertor.java	100% <0%> (+5.26%)	3% <0%> (ø)	⬇️
... and 3 more

[slievrly]

LGTM

[zhangthen]

+1