cmake_parse_arguments: Set variable if empty string given after keyword

If a single-value keyword is followed by an empty string, the
command unsets the variable for that keyword instead of setting
it to the empty string. This is inconsistent and unexpected. Add
policy CMP0174 which ensures the variable for a single-value
keyword is always set when any value is given, not just for a
non-empty value.

The new CMP0174 policy only affects the PARSE_ARGV form of
cmake_parse_arguments. The older form silently drops all empty
string arguments before processing the argument list.

Fixes: #25972

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