fix: don't use the globalThis shim when there are no shimmed globals

[dsherret]

With shims: {} (or every shim disabled), dnt still rewrote every unresolved globalThis to dntShim.dntGlobalThis and emitted an _dnt.shims.ts into the output, where the proxy merged globalThis with an empty object:

// in
export function isBrowser() {
  return typeof globalThis !== "undefined" && globalThis != null;
}
// out (before)
import * as dntShim from "./_dnt.shims.js";
export function isBrowser() {
    return typeof dntShim.dntGlobalThis !== "undefined" && dntShim.dntGlobalThis != null;
}

There's nothing to merge into globalThis in that case, so the rewrite is skipped when no shim declares any global names. The shim file is only emitted when something imports it, so it now disappears from the output as well.

Behaviour with shims configured is unchanged.

Closes #454