Request cache: Fix type error when deleting possibly undefined oldest entry

[tharropoulos]

Change Summary

Fixes a tsc error about oldestEntry potentially being undefined:

    const isCacheOverMaxSize = this.responseCache.size > maxSize;
    if (isCacheOverMaxSize) {
      const oldestEntry = this.responseCache.keys().next().value; // ❌ Argument of type 'string | undefined' is not assignable to parameter of type 'string'.

      this.responseCache.delete(oldestEntry);
    }

to just checking if it is defined, otherwise it just moves on:

      const oldestEntry = this.responseCache.keys().next().value;
      if (oldestEntry) {
        this.responseCache.delete(oldestEntry);
      }

This change tries to address the type error while not introducing any regressions by changing the underlying logic by throwing or returning early.

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