Implemented the "isNil" and "isNotNil" properties in the Optional+Nil extension.

Sources/Core/Extensions/Optional+Nil.swift

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftLibs open source project
+//
+// Copyright (c) 2023 Röck+Cöde VoF. and the SwiftLibs project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftLibs project authors
+//
+//===----------------------------------------------------------------------===//
+
+public extension Optional {
+    
+    // MARK: Properties
+    
+    /// A Boolean value indicating whether an optional has not been set.
+    var isNil: Bool { self == nil }
+    
+    /// A Boolean value indicating whether an optional has been set.
+    var isNotNil: Bool { !isNil }
+    
+}

Tests/Core/Cases/Extensions/Optional+NilTests.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftLibs open source project
+//
+// Copyright (c) 2023 Röck+Cöde VoF. and the SwiftLibs project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftLibs project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Core
+import XCTest
+
+final class Optional_NilTests: XCTestCase {
+
+    // MARK: Properties
+    
+    private var optional: Int?
+
+    private var result: Bool!
+    
+    // MARK: Tests
+    
+    func test_isNil_whenOptionalHasNotBeenSet() {
+        // GIVEN
+        // WHEN
+        result = optional.isNil
+        
+        // THEN
+        XCTAssertTrue(result)
+    }
+    
+    func test_isNil_whenOptionalHasBeenSet() {
+        // GIVEN
+        optional = 0
+        
+        // WHEN
+        result = optional.isNil
+        
+        // THEN
+        XCTAssertFalse(result)
+    }
+    
+    func test_isNotNil_whenOptionalHasNotBeenSet() {
+        // GIVEN
+        // WHEN
+        result = optional.isNotNil
+        
+        // THEN
+        XCTAssertFalse(result)
+    }
+    
+    func test_isNotNil_whenOptionalHasBeenSet() {
+        // GIVEN
+        optional = 0
+        
+        // WHEN
+        result = optional.isNotNil
+        
+        // THEN
+        XCTAssertTrue(result)
+    }
+
+}