[Bug] Fix null time dimension expression

[wcchang1115]

PR checklist

• [x] Ensure you have added or ran the appropriate tests for your PR.
• [x] DCO signed

What type of PR is this? bug

What this PR does / why we need it: expression of semantic model's time dimension could be optional

Which issue(s) this PR fixes: sc-32064

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov[bot]]

Codecov Report

Merging #877 (f1f7977) into main (a6b1e35) will increase coverage by 0.08%. Report is 11 commits behind head on main. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #877      +/-   ##
==========================================
+ Coverage   46.40%   46.49%   +0.08%     
==========================================
  Files          75       75              
  Lines       10337    10335       -2     
==========================================
+ Hits         4797     4805       +8     
+ Misses       5540     5530      -10     

Files Changed	Coverage Δ
piperider_cli/dbt/changeset.py	46.23% <ø> (+1.55%)	:arrow_up:
piperider_cli/cli.py	65.10% <100.00%> (ø)
piperider_cli/dbtutil.py	67.10% <100.00%> (ø)

... and 1 file with indirect coverage changes

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