Highlight NULL values with yellow background in query result cells for better visibility

Describe the bug
Currently, NULL values in the query result grid are displayed as plain text without any visual distinction. This makes it difficult to identify NULL values quickly when scrolling through large datasets.

To Reproduce
Steps to reproduce the behavior:

1. Execute any query that returns columns with NULL values in the result set.
2. Observe that NULL values are displayed without any background color.

Expected behavior
Cells containing NULL values should have a yellow background color to make them easily noticeable while navigating through the results.

Screenshots

VS Code version
Version: 1.106.2 (user setup)
Commit: 1e3c50d64110be466c0b4a45222e81d2c9352888
Date: 2025-11-19T16:56:50.023Z
Electron: 37.7.0
ElectronBuildId: 12781156
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Windows_NT x64 10.0.26100

Extension Logs

See: https://github.com/microsoft/vscode-pgsql?tab=readme-ov-file#usage-and-feedback

Add any other context about the problem here.

Start by reproducing the issue with a query that returns NULL values and inspect the query result grid entry point responsible for rendering result cells. Confirm that NULL cells receive a yellow background without changing other values, then verify the result grid in VS Code.