fix(charts): skip empty string filter values to prevent invalid datetime format

[sentry]

This PR addresses issue DALGO-BACKEND-2J7, where a DataError: (psycopg2.errors.InvalidDatetimeFormat) invalid input syntax for type date: "" occurred during chart data preview.

Root Cause:
The error was caused by empty string values ("") being passed as filter values for date columns. Specifically, when a user cleared a date filter, the frontend would send {"column": "date", "operator": "equals", "value": "", "data_type": "date"}. The backend's filter application logic in apply_dashboard_filters and apply_chart_filters did not adequately guard against empty strings, leading to SQL queries like WHERE date = '', which PostgreSQL rejects as an invalid date format.

Solution:
Implemented guards in ddpui/core/charts/charts_service.py within the apply_dashboard_filters and apply_chart_filters functions. These guards now check for None or empty/blank string values (value is None or (isinstance(value, str) and not value.strip())) and skip applying such filters to the SQL query. Special consideration was given to is_null and is_not_null operators, which legitimately operate without a value, ensuring they are not inadvertently skipped by the new guard.

Fixes DALGO-BACKEND-2J7

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.26%. Comparing base (11a5843) to head (35f94fd).

Files with missing lines	Patch %	Lines
ddpui/core/charts/charts_service.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1417      +/-   ##
==========================================
- Coverage   60.26%   60.26%   -0.01%     
==========================================
  Files         146      146              
  Lines       17222    17227       +5     
==========================================
+ Hits        10378    10381       +3     
- Misses       6844     6846       +2     

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