Fix storage hooks return types

[joedelia]

What is the current behavior, and the steps to reproduce the issue?

useSessionStorageValue with a defaultValue and no initializeWithValue returns typeof defaultValue | undefined.

What is the expected behavior?

It should not have undefined as a possible return type, as we have a defaultValue and initializeWithValue defaults to true.

How does this PR fix the problem?

We correctly check that initializeWithValue is actually false.

Checklist

• Have you read contribution guideline?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Is there an existing issue for this PR?
  • link issue here
• Have the files been linted and formatted?
• Have the docs been updated to match the changes in the PR?
• Have the tests been updated to match the changes in the PR?
• Have you run the tests locally to confirm they pass?

[joedelia]

Hey @xobotyi sorry I haven't created an issue first, just went ahead and checked the issue. Let me know if it's cool to merge or if we should fork for our org use! Thank you for maintaining this :)