beam_Release_Python_NightlySnapshot is broken due to directory not exist

:sdks:python:depSnapshot broke beam_Release_Python_NightlySnapshot due to:

sh: 1: cannot create /home/jenkins/jenkins-slave/workspace/beam_Release_Python_NightlySnapshot/src/sdks/python/build/requirements.txt: Directory nonexistent

This is affected by https://github.com/apache/beam/pull/9277. Directory `sdks/python/build` no longer exist when writes to sdks/python/build/requirements.txt. We can create an empty file first to fix this problem and do cleanup at same time if old file exists.