Issue 523 fix

[moisesjgomez]

Overview/Summary

1. Fixes issue 523 by allowing the disk size to be set to Null for automatically setting to size of provided image.

As part of this Pull Request I have

• [ ] Read the Contribution Guide and ensured this PR is compliant with the guide
• [ ] Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• [ ] Checked for duplicate Pull Requests
• [ ] Associated it with relevant GitHub Issues
• [ ] (AVD LZA Team Only) Associated it with relevant ADO Items
• [ ] Ensured my code/branch is up-to-date with the latest changes in the main branch
• [ ] Performed testing and provided evidence.
• [ ] Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Docs etc.)

[danycontre]

@moisesjgomez thanks for your contribution.