bugfix: NullPointerException in DB mock during CI

[l81893521]

Ⅰ. Describe what this PR did

When set the mock value null, would cause some NullPointerException during the CI.

Ⅱ. Does this pull request fix one issue?

#1916

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 #1917 into develop will decrease coverage by 0.02%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##             develop    #1917      +/-   ##
=============================================
- Coverage      55.26%   55.23%   -0.03%     
+ Complexity      2401     2399       -2     
=============================================
  Files            428      428              
  Lines          14219    14219              
  Branches        1664     1664              
=============================================
- Hits            7858     7854       -4     
- Misses          5658     5660       +2     
- Partials         703      705       +2

Impacted Files	Coverage Δ	Complexity Δ
...ource/sql/struct/cache/AbstractTableMetaCache.java	85.29% <0%> (-5.89%)	11% <0%> (ø)
...in/java/io/seata/server/session/GlobalSession.java	84.54% <0%> (-0.49%)	67% <0%> (-1%)
...o/seata/server/coordinator/DefaultCoordinator.java	48.01% <0%> (-0.4%)	27% <0%> (-1%)

[helloworlde]

LGTM

[slievrly]

LGTM

[xingfudeshi]

LGTM.