Merge pull request kubernetes#51229 from ixdy/always-create-vendor-BUILD

Automatic merge from submit-queue

Always create vendor/BUILD in hack/update-bazel.sh

**What this PR does / why we need it**: makes sure there's always a `vendor/BUILD` file.
When updating `godeps`, the [instructions](https://github.com/kubernetes/community/blob/master/contributors/devel/godep.md) say to recursively remove `vendor/`, which also removes the `vendor/BUILD` file. Unless you manually recreate this file, running `update-bazel.sh` would instead update the `all-srcs` rule in the root `BUILD.bazel` file, which is not desired. `gazelle` and `kazel` won't create `vendor/BUILD` on their own, since there are no go sources directly in `vendor/`. 

With this PR, we'll make sure that the `vendor/BUILD` file always exists, creating it if necessary.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes kubernetes#51075

**Release note**:

```release-note
NONE
```

/assign @spxtr @mikedanese 
cc @thockin