Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

tests/unit/infrastructure/RuntimeEnvironment/BrowserOs/BrowserOsDetector.spec.ts

@@ -15,7 +15,7 @@ describe('BrowserOsDetector', () => {
       const actual = sut.detect(userAgent);
       // assert
       expect(actual).to.equal(expected);
-    });
+    }, { excludeNull: true, excludeUndefined: true });
   });
   it('detects as expected', () => {
     BrowserOsTestCases.forEach((testCase) => {
@@ -27,7 +27,7 @@ describe('BrowserOsDetector', () => {
       expect(actual).to.equal(testCase.expectedOs, printMessage());
       function printMessage(): string {
         return `Expected: "${OperatingSystem[testCase.expectedOs]}"\n`
-          + `Actual: "${OperatingSystem[actual]}"\n`
+          + `Actual: "${actual === undefined ? 'undefined' : OperatingSystem[actual]}"\n`
           + `UserAgent: "${testCase.userAgent}"`;
       }
     });

tests/unit/infrastructure/RuntimeEnvironment/RuntimeEnvironment.spec.ts

@@ -16,7 +16,7 @@ describe('RuntimeEnvironment', () => {
         const absentWindow = absentValue;
         // act
         const act = () => createEnvironment({
-          window: absentWindow,
+          window: absentWindow as never,
         });
         // assert
         expect(act).to.throw(expectedError);
@@ -123,7 +123,7 @@ describe('RuntimeEnvironment', () => {
           // arrange
           const expectedValue = undefined;
           const windowWithAbsentOs = {
-            os: absentValue,
+            os: absentValue as never,
           };
           // act
           const sut = createEnvironment({

tests/unit/infrastructure/RuntimeEnvironment/WindowVariablesValidator.spec.ts

@@ -12,7 +12,7 @@ describe('WindowVariablesValidator', () => {
       itEachInvalidObjectValue((invalidObjectValue) => {
         // arrange
         const expectedError = 'window is not an object';
-        const window: Partial<WindowVariables> = invalidObjectValue;
+        const window: Partial<WindowVariables> = invalidObjectValue as never;
         // act
         const act = () => validateWindowVariables(window);
         // assert