Fix subset and superset for empty character set (#6000)

Fix isSubset() and isSuperset() involving the empty CharacterSet.

Author: MuhammadAnnaqeeb

stdlib/public/SDK/Foundation/CharacterSet.swift

@@ -436,6 +436,9 @@ public struct CharacterSet : ReferenceConvertible, Equatable, Hashable, SetAlgeb
 
     /// Returns true if `self` is a superset of `other`.
     public func isSuperset(of other: CharacterSet) -> Bool {
+        if other.isEmpty {
+            return true 
+        }
         return _mapUnmanaged { $0.isSuperset(of: other) }
     }
 

test/stdlib/TestCharacterSet.swift

@@ -241,6 +241,16 @@ class TestCharacterSet : TestCharacterSetSuper {
         expectEqual(0x6, bitmap[12])
         expectEqual(8192, bitmap.count)
     }
+    func test_superSetOfEmptySet(){
+        let emptySet = CharacterSet()
+        let immutableSet = CharacterSet(charactersIn:"abc")
+        expectTrue(emptySet.isSubset(of: immutableSet))
+        expectTrue(immutableSet.isSuperset(of: emptySet))
+        expectTrue(emptySet.isSubset(of: emptySet))
+        expectTrue(emptySet.isSuperset(of: emptySet))
+        expectFalse(emptySet.isSuperset(of: immutableSet))
+        expectFalse(immutableSet.isSubset(of: emptySet))
+    }
 }
 
 
@@ -264,6 +274,7 @@ CharacterSetTests.test("test_subtractNonEmptySet") { TestCharacterSet().test_sub
 CharacterSetTests.test("test_symmetricDifference") { TestCharacterSet().test_symmetricDifference() }
 CharacterSetTests.test("test_hasMember") { TestCharacterSet().test_hasMember() }
 CharacterSetTests.test("test_bitmap") { TestCharacterSet().test_bitmap() }
+CharacterSetTests.test("test_superSetOfEmptySet") { TestCharacterSet().test_superSetOfEmptySet() }
 runAllTests()
 #endif