pip_library: ensure pip_flags is always a string (#314)

When the `pip_flags` parameter and the `PipFlags` plugin configuration
option are both undefined, the output of `pip_library` contains the
[none](https://pypi.org/project/none/) module as well as the desired one
because `None` is erroneously passed as an argument to pip. Ensure
`None` is not passed to pip under these circumstances.

Fixes #313.

Author: chrisnovakovic

build_defs/python.build_defs

@@ -434,7 +434,7 @@ def pip_library(name:str, version:str, labels:list=[], hashes:list=None, package
     post_install_commands = post_install_commands or []
     use_pypi = CONFIG.PYTHON.USE_PYPI if use_pypi is None else use_pypi
     index_flag = '' if use_pypi else '--no-index'
-    pip_flags = pip_flags or CONFIG.PYTHON.PIP_FLAGS
+    pip_flags = pip_flags or CONFIG.PYTHON.PIP_FLAGS or ""
 
     repo_flag = ''
     repo = repo or CONFIG.PYTHON.DEFAULT_PIP_REPO