Always generate files in closure_grpc_web_library()

This adds a `ctx.actions.write` to each of the automatically generated source targets
that have been declared.  This prevents an issue if the source targets do not yield a
`.grpc.js` generated file because they do not contain an `rpc` definition (e.g. proto
files that only contain a `message`).

Resolves: grpc#1076

bazel/private/rules/closure_grpc_web_library.bzl

@@ -73,6 +73,7 @@ def _generate_closure_grpc_web_srcs(
         basename = src.basename[:-(len(src.extension) + 1)]
 
         js = actions.declare_file(basename + "_grpc_web_pb.js", sibling = src)
+        actions.write(output = js, content = "")
         files.append(js)
 
         _assert(
@@ -83,6 +84,7 @@ def _generate_closure_grpc_web_srcs(
 
         if "es6" == import_style:
             es6 = actions.declare_file(basename + ".pb.grpc-web.js", sibling = src)
+            actions.write(output = es6, content = "")
             es6_files.append(es6)
 
             _assert(root == es6.root.path, "ES6 file should have same root: '{}' != '{}'".format(root, es6.root.path))