fix(DatetimePicker): fix incorrect confirm value when v-model is not used

[chenjiangui]

fix(DatetimePicker): fix incorrect confirm value when v-model is not used (#9913）

[codecov-commenter]

Codecov Report

Merging #9938 (34b670d) into 2.x (2890b77) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              2.x    #9938   +/-   ##
=======================================
  Coverage   90.63%   90.63%           
=======================================
  Files         184      184           
  Lines        5212     5213    +1     
  Branches      307      307           
=======================================
+ Hits         4724     4725    +1     
  Misses        447      447           
  Partials       41       41           

Impacted Files	Coverage Δ
src/datetime-picker/DatePicker.js	99.15% <100.00%> (+<0.01%)	⬆️

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[chenjiahan]

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