Do not set an empty name on menu navigation landmarks

Problem/Motivation

block--system-menu-block.html.twig names each menu nav from its block label, since #3625205. It sets aria-label even when the label is empty, which gives an unlabelled placement a landmark with an empty name.

Separately, the homepage places the main menu three times (mobile menu, a header section, the footer), all labelled "Main navigation", so axe reports landmark-unique. Those labels are block placement config in the RightUp recipe, so unique names belong there.

Steps to reproduce

1. Place a system menu block with an empty label.
2. The rendered nav carries an empty aria-label.

Proposed resolution

Set aria-label only when the block label is not empty. This does not clear the landmark-unique failure on its own; that needs distinct labels in the recipe.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ❌ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• None visible.

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A

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