Fix the crash caused by a nullptr variable

[xtcyclist]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Close https://github.com/vesoft-inc/nebula-ent/issues/1971
Close #5101

Description:

ex->what() is called without checking whether ex is nullptr.

How do you solve it?

For this else body in particular, no need to call ex->what().

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....

[codecov-commenter]

Codecov Report

Base: 76.96% // Head: 77.82% // Increases project coverage by +0.85% 🎉

Coverage data is based on head (2ef9442) compared to base (8e7aa68).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5100      +/-   ##
==========================================
+ Coverage   76.96%   77.82%   +0.85%     
==========================================
  Files        1108     1108              
  Lines       82405    82420      +15     
==========================================
+ Hits        63425    64141     +716     
+ Misses      18980    18279     -701     

Impacted Files	Coverage Δ
src/clients/storage/StorageClientBase-inl.h	69.33% <0.00%> (-7.38%)	⬇️
src/graph/session/GraphSessionManager.h	87.50% <0.00%> (-12.50%)	⬇️
src/storage/BaseProcessor-inl.h	75.26% <0.00%> (-7.53%)	⬇️
src/graph/executor/StorageAccessExecutor.h	52.27% <0.00%> (-6.82%)	⬇️
src/graph/executor/query/TraverseExecutor.h	90.00% <0.00%> (-6.43%)	⬇️
src/clients/storage/StorageClientBase.h	56.45% <0.00%> (-3.23%)	⬇️
src/storage/mutate/AddEdgesProcessor.cpp	78.62% <0.00%> (-3.06%)	⬇️
src/parser/GraphScanner.h	92.63% <0.00%> (-2.00%)	⬇️
src/graph/executor/query/ProjectExecutor.cpp	61.11% <0.00%> (-1.75%)	⬇️
... and 68 more

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[kikimo]

LGTM