fix: check labelNode existed before change postion

[NewByVector]

Description

ref: #3601
在修改 label position 之前检测 label node 是否存在

Motivation and Context

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Enhancement (changes that improvement of current feature or performance)
• Refactoring (changes that neither fixes a bug nor adds a feature)
• Test Case (changes that add missing tests or correct existing tests)
• Code style optimization (changes that do not affect the meaning of the code)
• Docs (changes that only update documentation)
• Chore (changes that don't modify src or test files)

Self Check before Merge

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #3737 (5022afe) into master (c70021f) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3737   +/-   ##
=======================================
  Coverage   10.36%   10.36%           
=======================================
  Files         180      180           
  Lines       10498    10498           
  Branches     2555     2555           
=======================================
  Hits         1088     1088           
  Misses       9321     9321           
  Partials       89       89           

Flag	Coverage Δ
x6	10.36% <0.00%> (ø)

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Impacted Files	Coverage Δ
packages/x6/src/view/edge.ts	1.13% <0.00%> (ø)