[VAULT-4034] Revert back to caching nil values (#13013)

* Revert "[VAULT-4034] Only cache non-nil values (#12993)"

This reverts commit 67e1ed06c7.

* Update sdk/physical/cache.go

Co-authored-by: Nick Cabatoff <ncabatoff@hashicorp.com>

Co-authored-by: Nick Cabatoff <ncabatoff@hashicorp.com>

changelog/12993.txt

@@ -1,3 +0,0 @@
-```release-note:bug
-sdk/physical: Fix to only populate cache with non-nil values
-```

sdk/physical/cache.go

@@ -184,10 +184,8 @@ func (c *Cache) Get(ctx context.Context, key string) (*Entry, error) {
 		return nil, err
 	}
 
-	if ent != nil {
+	// Cache the result, even if nil
-		// Cache the result
 	c.lru.Add(key, ent)
-	}
 
 	return ent, nil
 }

sdk/physical/inmem/cache_test.go

@@ -328,45 +328,3 @@ func TestCache_Refresh(t *testing.T) {
 		t.Fatalf("expected value baz, got %s", string(r.Value))
 	}
 }
-
-// TestCache_UpdateFromStorage fetches a nil value from cache, updates
-// the underlying storage and checks the cache value on a subsequent lookup
-func TestCache_UpdateFromStorage(t *testing.T) {
-	logger := logging.NewVaultLogger(log.Debug)
-
-	inm, err := NewInmem(nil, logger)
-	if err != nil {
-		t.Fatal(err)
-	}
-	cache := physical.NewCache(inm, 0, logger, &metrics.BlackholeSink{})
-	cache.SetEnabled(true)
-
-	ent := &physical.Entry{
-		Key:   "foo",
-		Value: []byte("bar"),
-	}
-
-	// Read should return nil
-	out, err := cache.Get(context.Background(), "foo")
-	if err != nil {
-		t.Fatalf("err: %v", err)
-	}
-	if out != nil {
-		t.Fatalf("should not have key")
-	}
-
-	// Add data to the underlying backend
-	inm.Put(context.Background(), ent)
-
-	// Read should return data
-	out, err = cache.Get(context.Background(), "foo")
-	if err != nil {
-		t.Fatalf("err: %v", err)
-	}
-	if out == nil {
-		t.Fatalf("should have key")
-	}
-	if string(out.Value) != "bar" {
-		t.Fatalf("expected value bar, got %s", string(out.Value))
-	}
-}