Update validators.cs.xlf

fixed CZ translation.
there is not such a word as "null" in czech
minor #9763 Update validators.cs.xlf (gondo)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9763).

Discussion
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Update validators.cs.xlf

fixed CZ translation.
there is not such a word as "null" in czech

Commits
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eac9032 Update validators.cs.xlf