fix: detached glass shows 'null' in header when pane has no title

[ohxyz]

Problem

Detaching a glass from a pane that has no title rendered the literal text null in the detached glass's header.

Root cause

Glass.build() only created a bw-glass-title element when a title was present. So when detaching a titleless pane:

• The source pane had no bw-glass-title → paneEl.querySelector('bw-glass-title') returned null.
• The newly created detached glass also had no title element → detachedGlass.titleElement was null.
• The fallback branch ran titleEl.replaceWith(glassTitleEl) where glassTitleEl was null. ChildNode.replaceWith(null) coerces null to the string "null", inserting it as a visible text node.

Fix

• Glass now always creates a bw-glass-title element, leaving it empty when no title is passed.
• With the title element guaranteed to exist, the detach handler's null-title fallback branch is dead code and is removed.

Test plan

• Detach a pane with a title → title carries over as before.
• Detach a pane without a title → empty header, no null text.
• dev/features/bwin-detached-glass.js: the bottom pane is now titleless to exercise this case manually.