Fix crash when deploying zero functions. (#3520)

Previously most code read the desired backend from
`options.config.get("functions.backend")` which was set to the
empty backend correctly. Code that depended on payload.functions.backend
crashed because payload.functions was null when the backend was
empty.

Since optins.config should be firebase.json data, this change
normalizes on payload.functions.backend and ensures that it
is never null while options.config.get('functions') is present
(i.e. when the customer has functions to deploy).

src/deploy/functions/deploy.ts

@@ -58,7 +58,7 @@ export async function deploy(
   }
 
   try {
-    const want = options.config.get("functions.backend") as backend.Backend;
+    const want = payload.functions!.backend;
     const uploads: Promise<void>[] = [];
     if (want.cloudFunctions.some((fn) => fn.apiVersion === 1)) {
       uploads.push(uploadSourceV1(context));

src/deploy/functions/prepare.ts

@@ -54,7 +54,7 @@ export async function prepare(
 
   logger.debug(`Analyzing ${runtimeDelegate.name} backend spec`);
   const wantBackend = await runtimeDelegate.discoverSpec(runtimeConfig, env);
-  options.config.set("functions.backend", wantBackend);
+  payload.functions = { backend: wantBackend };
   if (backend.isEmptyBackend(wantBackend)) {
     return;
   }
@@ -91,11 +91,6 @@ export async function prepare(
     })
   );
 
-  // Build a regionMap, and duplicate functions for each region they are being deployed to.
-  payload.functions = {
-    backend: wantBackend,
-  };
-
   // Validate the function code that is being deployed.
   validate.functionIdsAreValid(wantBackend.cloudFunctions);