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.

src/infrastructure/RuntimeEnvironment/BrowserOs/DetectorBuilder.ts

@@ -25,9 +25,9 @@ export class DetectorBuilder {
     };
   }
 
-  private detect(userAgent: string): OperatingSystem {
+  private detect(userAgent: string): OperatingSystem | undefined {
     if (!userAgent) {
-      throw new Error('missing userAgent');
+      return undefined;
     }
     if (this.existingPartsInUserAgent.some((part) => !userAgent.includes(part))) {
       return undefined;

src/infrastructure/RuntimeEnvironment/RuntimeEnvironment.ts

@@ -37,7 +37,7 @@ export class RuntimeEnvironment implements IRuntimeEnvironment {
   }
 }
 
-function getUserAgent(window: Partial<Window>): string {
+function getUserAgent(window: Partial<Window>): string | undefined {
   return window?.navigator?.userAgent;
 }