bugfix: fix NPE in UndoExecutorFactory

[booogu]

Ⅰ. Describe what this PR did

fix a minor NPE bug:
Swapped the order of constant and variable in a equals() method call

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1902 into develop will increase coverage by 0.01%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop    #1902      +/-   ##
=============================================
+ Coverage      55.41%   55.43%   +0.01%     
- Complexity      2406     2410       +4     
=============================================
  Files            428      428              
  Lines          14346    14346              
  Branches        1699     1699              
=============================================
+ Hits            7950     7952       +2     
  Misses          5679     5679              
+ Partials         717      715       -2

Impacted Files	Coverage Δ	Complexity Δ
.../seata/rm/datasource/undo/UndoExecutorFactory.java	64.28% <0%> (ø)	3 <3> (ø)	⬇️
...o/seata/server/coordinator/DefaultCoordinator.java	48.41% <0%> (+0.39%)	30% <0%> (+3%)	⬆️
...in/java/io/seata/server/session/GlobalSession.java	85.02% <0%> (+0.48%)	68% <0%> (+1%)	⬆️

[xingfudeshi]

LGTM.

[slievrly]

LGTM