themeOverrides with `undefined` values result in Error during render.

[atamon]

Hello,

First of all thanks for developing this project. We use this in production to implement a minimal spreadsheet-like preview and it works great!

We recently bumped the dependency of @glideapps/glide-data-grid and @glideapps/glide-data-grid-source from 5.3.2 to 6.0.4-alpha24 and got this "crash":

color-parser.js:17 Uncaught TypeError: Cannot read properties of undefined (reading 'toLowerCase')
    at parseToRgba (color-parser.js:17:1)
    at blend (color-parser.js:67:1)
    at mergeAndRealizeTheme (styles.js:109:1)
    at data-grid-render.header.js:33:1
    at walkColumns (data-grid-render.walk.js:43:1)
    at drawGridHeaders (data-grid-render.header.js:22:1)
    at drawHeaderTexture (data-grid-render.js:147:1)
    at drawGrid (data-grid-render.js:231:1)
    at data-grid.js:393:1
    at data-grid.js:451:1

Which looks to end up at this line:

glide-data-grid/packages/core/src/internal/data-grid/color-parser.ts

Line 23 in 0875d78

const normalizedColor = color.toLowerCase().trim();

Our offending code/repro defined a themeOverride like this on the GridColumn type:

  themeOverride: {
    bgCell: visualizeHidden ? 'lightgrey' : undefined,
    bgHeader: visualizeHidden ? 'lightgrey' : undefined,
  }

It's of course quite easy to get around this issue for us by filtering out undefined properties from the themeOverride. But I wanted to give you the chance to fix this if you consider it a bug in the new major.

Please close this issue if you believe this is the intended behavior going forwards.