Fix blank module handling in mbc build and bump alpine base in dockerfile

Dockerfile

@@ -3,7 +3,7 @@ FROM node:10 AS frontend-builder
 COPY ./maubot/management/frontend /frontend
 RUN cd /frontend && yarn --prod && yarn build
 
-FROM alpine:3.8
+FROM alpine:3.9
 
 ENV UID=1337 \
     GID=1337

maubot/cli/commands/build.py

@@ -88,7 +88,7 @@ def write_plugin(meta: PluginMeta, output: Union[str, IO]) -> None:
         for module in meta.modules:
             if os.path.isfile(f"{module}.py"):
                 zip.write(f"{module}.py")
-            elif os.path.isdir(module):
+            elif module is not None and os.path.isdir(module):
                 zipdir(zip, module)
             else:
                 print(Fore.YELLOW + f"Module {module} not found, skipping" + Fore.RESET)