refactor(ivy): do not generate providedIn: null (#34116)

We should only generate the `providedIn` property in injectable
defs if it has a non-null value. `null` does not communicate
any information to the runtime that isn't communicated already
by the absence of the property.

This should give us some modest code size savings.

PR Close #34116

aio/scripts/_payload-limits.json

@@ -12,7 +12,7 @@
     "master": {
       "uncompressed": {
         "runtime-es2015": 2987,
-        "main-es2015": 462449,
+        "main-es2015": 461697,
         "polyfills-es2015": 52503
       }
     }