bugfix: fix the problem of building undoLog exception when update join does not update data

[GoodBoyCoder]

• I have registered the PR changes.

Ⅰ. Describe what this PR did

If the before and agter images are both empty, skip prepareUndoLog directly

Ⅱ. Does this pull request fix one issue?

fixes #6976

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

Ⅳ. Describe how to verify it

make an updateJoin statement with an empty query result to test

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 52.65%. Comparing base (60a81b1) to head (9aaa658).
Report is 2 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...datasource/exec/mysql/MySQLUpdateJoinExecutor.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x    #6994   +/-   ##
=========================================
  Coverage     52.65%   52.65%           
- Complexity     6571     6576    +5     
=========================================
  Files          1126     1126           
  Lines         39942    39945    +3     
  Branches       4680     4682    +2     
=========================================
+ Hits          21031    21034    +3     
- Misses        16911    16913    +2     
+ Partials       2000     1998    -2     

Files with missing lines	Coverage Δ
...datasource/exec/mysql/MySQLUpdateJoinExecutor.java	78.26% <75.00%> (-0.23%)	⬇️

... and 4 files with indirect coverage changes

[funky-eyes]

LGTM