make: make ParseWord in condition parser simpler

Merge the two return values (bool, string) into a single return value.
As before, the caller cannot observe the difference between a parse
error and an empty word, both are handled in the same way.

In CondParser_ComparisonOrLeaf, the word cannot be empty since the
calling function CondParser_Token already handles all cases that could
lead to an empty word.

No functional change.

(rillig)