fix(table): fix filtering when table is not sorted

[dauriata]

Proposed Changes

Fixes table filtering when table is not yet sorted
currentSortColumn is undefined when the table is not sorted ( no defaultSort prop defined) and the property is accessed in handleFilter function throwing an error
Adding a safe guard in the function

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 17.85%. Comparing base (484cfe8) to head (71b7800).
Report is 57 commits behind head on develop.

Files	Patch %	Lines
packages/oruga/src/components/table/Table.vue	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #911       +/-   ##
============================================
- Coverage    56.71%   17.85%   -38.86%     
============================================
  Files           30      292      +262     
  Lines         1511     7319     +5808     
  Branches       544     2095     +1551     
============================================
+ Hits           857     1307      +450     
- Misses         654     5061     +4407     
- Partials         0      951      +951     

Flag	Coverage Δ
oruga-next	17.85% <0.00%> (-38.86%)	⬇️

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