tpl: Fix partial decorator panic when partial returns falsy

tpl: Fix partial decorator panic when partial returns falsy

When {{ with partial "foo" }} returns a falsy value (false, nil, ""),
the with block is skipped but _popPartialDecorator was only called
inside the with block. This left an orphan entry on the decorator
stack, causing "partial decorator ID mismatch" panic on subsequent
partial calls.

Add _popPartialDecorator call to the else branch to ensure the
decorator stack is always balanced regardless of the partial's
return value.

Fixes #14419