floss: Fix env encoding issue with build.py

Running commands using subprocess can fail if there are any `None`
values in env as os.fsencode throws a TypeError. Manually replace them
with empty strings in that case.

Bug: 356870224
Test: mmm packages/modules/Bluetooth
Change-Id: I6ce9e6e79de073686b102e8c783981f0145b9a4e