common: new utility function: DerefOrDefault()

This is sort of the opposite of ToPtr().  It dereferences a pointer to
its base value or returns the default value for the type if the pointer
is nil.

internal/common/pointers.go

@@ -3,3 +3,13 @@ package common
 func ToPtr[T any](x T) *T {
 	return &x
 }
+
+// DerefOrDefault returns the dereferenced value of the given pointer or the
+// default value for the type if unset.
+func DerefOrDefault[T any](p *T) T {
+	var v T
+	if p != nil {
+		v = *p
+	}
+	return v
+}