[data grid] Fix CSV export for null and undefined values

[k-rajat19]

Fixes #14082

this issue comes from the changes done in #13560 , the motive of that PR is to convert any non-string value into a string using template literals but it also converts null and undefined values into strings which then exported to CSV as a string.
I have added a check for these values.

• I have followed (at least) the PR section of the contributing guide.

[k-rajat19]

not sure if this check should stay here or not, it probably not
before #13560 this check was useful to opt out of null and undefined values

mui-x/packages/x-data-grid/src/hooks/features/export/serializers/csvSerializer.ts

Lines 79 to 80 in 38184fe

	if (value === null || value === undefined) {
	this.rowString += '';

[arminmeh]

not sure if this check should stay here or not, it probably not before #13560 this check was useful to opt out of null and undefined values

mui-x/packages/x-data-grid/src/hooks/features/export/serializers/csvSerializer.ts

Lines 79 to 80 in 38184fe

	if (value === null || value === undefined) {
	this.rowString += '';

I have removed this, thanks for pointing it out

[arminmeh]

I have added a test to the PR, to prevent future regressions.
Thanks for the contribution @k-rajat19

[mui-bot]

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