fix: guard against null entry array in wrenMapFind for empty maps

Add early return of UINT32_MAX when map capacity is zero in wrenMapFind,
preventing a crash from modulo operation on zero during hash index calculation.
Includes regression test verifying subscript access on empty map returns null.

test/map/subscript_empty_map.wren

@@ -0,0 +1,4 @@
+// This is a regression test to ensure map handles a null entry array.
+
+var map = {}
+IO.print(map["key"]) // expect: null