refactor: make index option nullable for browser-esbuild

For testing use cases, we don't need an `index.html` file in the same capacity as a typical application. The builder already omits an `index.html` page when not set, this just updates the schema to reflect that.

The parameter could be left optional, rather than required but allowing `false`. However doing it this way prevents users from accidentally forgetting to provide an index while still allowing users to explicitly disable index generation. We use `false` instead of `null` so users can write `--no-index` on the command line and get the same behavior, which would not be possible with `null`.

(cherry picked from commit 0d4a40f)

Author: dgp1130

packages/angular_devkit/build_angular/src/builders/browser-esbuild/schema.json

@@ -397,6 +397,10 @@
             }
           },
           "required": ["input"]
+        },
+        {
+          "const": "false",
+          "description": "Does not generate an `index.html` file."
         }
       ]
     },