plugins: meson: IntrospectJob: if the build dir is invalid, try to get it from the project

When importing a meson.build with a previously configured build directory, introspect job fails with an invalid directory. The problem occurs because the introspect job gets created when the project build directory is null. The introspect job caches the null build directory and then 'builder()->configure(project)' sets the project build directory to a valid directory.