Add workaround for nil detection issue in Xcode 16

Sources/AnyExpression.swift

@@ -747,13 +747,21 @@ extension AnyExpression {
 
     // Test if a value is nil
     static func isNil(_ value: Any) -> Bool {
-        if let optional = value as? _Optional {
+        switch value {
+        case let optional as _Optional:
             guard let value = optional.value else {
                 return true
             }
             return isNil(value)
+        case is NSNull:
+            return true
+        case is _String:
+            // Avoid treating "nil" as nil
+            return false
+        default:
+            // Workaround for _Optional not working in Xcode 16
+            return String(describing: value) == "nil"
         }
-        return value is NSNull
     }
 
     // Test if a value supports subscripting

Tests/AnyExpressionTests.swift

@@ -1871,6 +1871,21 @@ class AnyExpressionTests: XCTestCase {
         XCTAssertNil(try expression.evaluate() as String?)
     }
 
+    func testStringNilNotTreatedAsNil() {
+        let expression = AnyExpression("'nil'")
+        XCTAssertNotNil(try expression.evaluate() as String?)
+    }
+
+    func testNSStringNilNotTreatedAsNil() {
+        let expression = AnyExpression("foo", constants: ["foo": "nil" as NSString])
+        XCTAssertNotNil(try expression.evaluate() as String?)
+    }
+
+    func testSubstringNilNotTreatedAsNil() {
+        let expression = AnyExpression("foo", constants: ["foo": Substring("nil")])
+        XCTAssertNotNil(try expression.evaluate() as String?)
+    }
+
     // MARK: Errors
 
     func testUnknownOperator() {