Stencil-to-Lit migration unintentionally widened several interface properties to allow null (e.g. IgcSplitPane.panes)

[igdmdimitrov]

Stencil-to-Lit migration unintentionally widened several interface properties to allow null.

Example in IgcSplitPane:

• Before: panes: IgcDockManagerPane[];
• After: panes: IgcDockManagerPane[] | null;

Impact:

• No functional/runtime impact — these fields are never actually assigned null during normal Dock Manager operation.
• Type-level regression only: consumers with strictNullChecks may now get unnecessary null-narrowing requirements, and the public docs/types are inconsistent with actual behavior.