custom_build should allow 0 deps

[nicks]

In our current Bazel doc, we give the example:

custom_build(
  'bazel/snack',
  'bazel run //snack:image',
  [],
  tag="image",
)

This is currently an error:

Error: Argument 3 (deps) can't be empty

I don't see any good reason why this should be an error - it should be OK to produce an image if you don't have the source code checked out.