Fix invalid memory error for python binary build

The root cause is we didn't check if the optionalpath is valid or not,
the registerbuildation function directly invoke the path var and
cause the invalid memory error. We just return if the launcher doesn't
exist.

The ctx.VisitDirectDepsWithTag() also handles allowmissingdependency so
we are ok if launcher doesn't exist.

Test: N/A
Bug: b/116698229, b/67510844
Change-Id: I40941079a64d7797ab879fc5edaa29e835b493a0