tool/var: also error when expansion result starts with NUL

Expansions whose output starts with NUL were being expanded to the empty
string, and not being recognised as values that contai

tool/var: also error when expansion result starts with NUL

Expansions whose output starts with NUL were being expanded to the empty
string, and not being recognised as values that contain a NUL byte, and
should error.

Closes #11694

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