[iOS] Set height for EmptyView if is not an IView

[rmarinho]

Description of Change

We introduced some calculations for when EmptyView is a IView, but the user can just provide a string as the EmptyView and that will be rendered as UILabel on iOS.
By setting by default the CollectionView.Height we keep the old behaviour.

Issues Fixed

Fixes #26663

[Copilot]

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src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25551.cs:11

• The namespace was changed from 'Microsoft.Maui.TestCases.Tests.Tests.Issues' to 'Microsoft.Maui.TestCases.Tests.Issues'. Verify if this change aligns with the project's namespace conventions.

namespace Microsoft.Maui.TestCases.Tests.Issues

[Copilot]

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.