r\shared_image: Fix update issue related to unchanged eula

[myc2h6o]

• Fix Failure to add/change tags on imported azurerm_shared_image (Error: Changing property 'galleryImage.properties.eula' is not allowed.) #16792
• eula cannot be changed, mark it as ForceNew
• even empty, eula cannot be specified as empty string in an update call, so adding it only when it's not empty.
• This is for quick fix for the specified issue, furthermore, I think we should split the Create and Update of this resource, will do that in a separate pr

[katbyte]

LGTM 🦖

[github-actions]

This functionality has been released in v3.7.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.