feat: splitOn_nil to use BEq instead of DecidableEq (#31679)

Generalizes this theorem with a quick change.

Author: BoltonBailey

Mathlib/Data/List/SplitOn.lean

@@ -14,7 +14,7 @@ variable {α : Type*} (p : α → Bool) (xs : List α) (ls : List (List α))
 attribute [simp] splitAt_eq
 
 @[simp]
-theorem splitOn_nil [DecidableEq α] (a : α) : [].splitOn a = [[]] :=
+theorem splitOn_nil [BEq α] (a : α) : [].splitOn a = [[]] :=
   rfl
 
 @[simp]