hgweb: handle None from templatedir() equally bad in webcommands.py

The following paragraph is based just on my reading of the code; I
have not tried to test it.

Before my recent work on templates in frozen binaries, it seems both
`hgwebdir_mod.py` and `webcommands.py` would pass in an empty list
into `staticfile()` when running in a frozen binary. That would then
result in a variable in that function (`path`) not getting bound
before its first use. I then changed that without thinking in D8786 so
we passed a `None` value into the function, which made it break in
another way (trying to iterate over `None`). Then I tried to fix it up
in D8810, but I only changed `hgwebdir_mod.py` for some reason, and it
still doesn't actually work in frozen binaries (which seems fair,
since was broken before my changes too).

This patch just replicates the half-assed "fix" from D8810 in
`webcommands.py`, so they look more similar so I can start refactoring
them in the same way.

Differential Revision: https://phab.mercurial-scm.org/D8933

mercurial/hgweb/webcommands.py

@@ -1320,7 +1320,8 b' def static(web):'
     static = web.config(b"web", b"static", untrusted=False)
     if not static:
         tp = web.templatepath or templater.templatedir()
-        static = os.path.join(tp, b'static')
+        if tp is not None:
+            static = os.path.join(tp, b'static')
 
     staticfile(static, fname, web.res)
     return web.res.sendresponse()