Fix term_exists() for parent = 0.

Passing a 0 (or '0') as the 'parent' param of term_exists() should limit
results to terms with no parent.

Adds unit test.

Fixes #29851.

git-svn-id: https://develop.svn.wordpress.org/trunk@29863 602fd350-edb4-49c9-b593-d223f7449a82