bugfix:fix NPE when use UNION ALL in select statement.

[xingfudeshi]

Ⅰ. Describe what this PR did

fix NPE when use UNION ALL in select statement.

Ⅱ. Does this pull request fix one issue?

fixes #794

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

i have done integration testing.

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #891 into develop will decrease coverage by 0.19%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             develop     #891     +/-   ##
============================================
- Coverage      38.53%   38.33%   -0.2%     
+ Complexity      1035     1033      -2     
============================================
  Files            222      222             
  Lines           8662     8662             
  Branches        1048     1048             
============================================
- Hits            3338     3321     -17     
- Misses          4931     4948     +17     
  Partials         393      393

Impacted Files	Coverage Δ	Complexity Δ
.../io/seata/rm/datasource/sql/SQLVisitorFactory.java	33.33% <100%> (ø)	2 <0> (ø)	⬇️
...ain/java/io/seata/core/rpc/netty/ShutdownHook.java	56.25% <0%> (-25%)	6% <0%> (-2%)
...eata/core/rpc/netty/AbstractRpcRemotingClient.java	17.22% <0%> (-3.35%)	7% <0%> (ø)
...o/seata/server/coordinator/DefaultCoordinator.java	53.47% <0%> (-1.39%)	17% <0%> (ø)

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[CoffeeLatte007]

LGTM

[slievrly]

LGTM