text: Title: Dashtable case-insensitive filter causes exception when the column contains null value
  Body: If a column in the table contains null value at some rows and you try to do a case insensitive filtering like "ine foo', a javascript exception "Cannot read property 'toString' of null" exception" will occur. Apparently it is caused by the line "lhs.toString().toUpperCase()" of fnEval() method in relational.ts failed to check whether lhs (i.e. the cell value) is null or not.
  
  A sample app to reproduce the problem.
  
  ```
  from dash import html
  from dash import dash_table
  import pandas as pd
  from collections import OrderedDict
  
  import dash
  
  app = dash.Dash(__name__)
  
  df = pd.DataFrame(OrderedDict([
      ('climate', [None, 'Snowy', 'Sunny', 'Rainy']),
      ('temperature', [13, 43, 50, 30]),
      ('city', ['NYC', None, 'Miami', 'NYC'])
  ]))
  
  
  app.layout = html.Div([
      dash_table.DataTable(
          id='table',
          data=df.to_dict('records'),
          columns=[
              {'id': 'climate', 'name': 'climate'},
              {'id': 'temperature', 'name': 'temperature'},
              {'id': 'city', 'name': 'city'},
          ],
          filter_action="native",
      ),
      html.Div(id='table-dropdown-container')
  ])
  
  
  if __name__ == '__main__':
      app.run_server(debug=True, port=8051)
  ```
  Run the app, in the 'city' column header of the table, type in 'ine foo' and hit enter, which should reproduce the problem.
  
  I had a PR for fixing this bug for an old version of dash-table at https://github.com/plotly/dash-table/pull/935/files
  
  Environment:
  ```
  dash                 2.17.1
  dash-core-components 2.0.0
  dash-html-components 2.0.0
  dash-table           5.0.0
  
  ```
      - OS: Ubuntu 22.04
      - Browser: Chrome
      - Version: 127.0.6533.119
  
  ![image](https://github.com/user-attachments/assets/a49e1332-8cce-4783-ae37-994a8aeb2477)