fix: unembed edge only when have new parent

[NewByVector]

Description

Close #2631

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 #3773 (b42af30) into master (8ecfc86) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3773   +/-   ##
=======================================
  Coverage   10.35%   10.35%           
=======================================
  Files         180      180           
  Lines       10503    10503           
  Branches     2556     2556           
=======================================
  Hits         1088     1088           
  Misses       9326     9326           
  Partials       89       89           

Flag	Coverage Δ
x6	10.35% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/x6/src/model/edge.ts	7.59% <0.00%> (ø)