fix: handle empty collection in binarySearch to prevent crash

[Nepomuk5665]

Summary

This PR fixes a runtime crash in LLBCASFileTree.binarySearch when called with an empty collection.

The Bug

The current implementation immediately calls elements.index(before: elements.endIndex) on line 16 without checking if the collection is empty. For an empty collection, startIndex == endIndex, and calling index(before: endIndex) is undefined behavior that causes a crash.

var lo: C.Index = elements.startIndex
var hi: C.Index = elements.index(before: elements.endIndex)  // CRASH if empty!

The Fix

Added an early guard clause to return nil for empty collections before attempting to compute indices:

guard !elements.isEmpty else { return nil }

This is the correct semantic behavior - searching for an element in an empty collection should return nil (not found), not crash.

Impact

• Prevents runtime crash when binary search is called with empty input
• No behavioral change for non-empty collections
• Consistent with Swift standard library conventions where searching empty collections returns nil/not found