hotdoc module: fix broken include paths

Since commit 32b14b1bb5, hotdoc is run
during configure as an external program. It turns out though, that in
some cases we passed NoneType in the cmd array, previously to
hotdoc.run_hotdoc.run() and now via subprocesses. The former "worked"
due to ignoring unknown arguments (?) but the latter was broken because
command line interfaces don't accept python NoneType objects, naturally.

End result: when for example building Meson's own documentation, this
fails with a python traceback.

The reason this happens to begin with turns out to be, once again,
because of the legacy debt of homebrewed kwargs parsing. We have a
function for `process_known_args` that handles args and ignores them if
they are NoneType, and then include_paths is handled via a custom
processor that internally adds them, then returns a *list* of NoneType
which then gets appended to the global cmd, because the logic ends up as
`[None, None] is None` which is a failed check, so we go ahead and add
it.

It's odd that we ever attempted to process it twice to begin with, so
let's simply not do that.

mesonbuild/modules/hotdoc.py

@@ -283,8 +283,7 @@ class HotdocTargetBuilder:
         self.process_known_arg("--project-version")
         self.process_known_arg("--sitemap", value_processor=self.ensure_file)
         self.process_known_arg("--html-extra-theme", value_processor=self.ensure_dir)
-        self.process_known_arg(None, "include_paths",
-                               value_processor=lambda x: [self.include_paths.add(self.ensure_dir(v)) for v in x])
+        self.include_paths.update(self.ensure_dir(v) for v in self.kwargs.pop('include_paths'))
         self.process_known_arg('--c-include-directories', argname="dependencies", value_processor=self.process_dependencies)
         self.process_gi_c_source_roots()
         self.process_extra_assets()