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/EnvironmentVariables/EnvironmentVariablesFactory.ts

@@ -5,6 +5,7 @@ import { itIsSingleton } from '@tests/unit/shared/TestCases/SingletonTests';
 import { EnvironmentVariablesFactory, EnvironmentVariablesValidator } from '@/infrastructure/EnvironmentVariables/EnvironmentVariablesFactory';
 import { ViteEnvironmentVariables } from '@/infrastructure/EnvironmentVariables/Vite/ViteEnvironmentVariables';
 import { IEnvironmentVariables } from '@/infrastructure/EnvironmentVariables/IEnvironmentVariables';
+import { expectExists } from '@tests/shared/Assertions/ExpectExists';
 
 describe('EnvironmentVariablesFactory', () => {
   describe('instance', () => {
@@ -23,7 +24,7 @@ describe('EnvironmentVariablesFactory', () => {
   });
   it('validates its instance', () => {
     // arrange
-    let validatedInstance: IEnvironmentVariables;
+    let validatedInstance: IEnvironmentVariables | undefined;
     const validatorMock = (instanceToValidate: IEnvironmentVariables) => {
       validatedInstance = instanceToValidate;
     };
@@ -31,6 +32,7 @@ describe('EnvironmentVariablesFactory', () => {
     const sut = new TestableEnvironmentVariablesFactory(validatorMock);
     const actualInstance = sut.instance;
     // assert
+    expectExists(validatedInstance);
     expect(actualInstance).to.equal(validatedInstance);
   });
   it('throws error if validator fails', () => {

tests/unit/infrastructure/EnvironmentVariables/EnvironmentVariablesValidator.spec.ts

@@ -1,5 +1,4 @@
 import { describe, it, expect } from 'vitest';
-import { itEachAbsentObjectValue } from '@tests/unit/shared/TestCases/AbsentTests';
 import { EnvironmentVariablesStub } from '@tests/unit/shared/Stubs/EnvironmentVariablesStub';
 import { validateEnvironmentVariables } from '@/infrastructure/EnvironmentVariables/EnvironmentVariablesValidator';
 import { IEnvironmentVariables } from '@/infrastructure/EnvironmentVariables/IEnvironmentVariables';
@@ -28,17 +27,6 @@ describe('EnvironmentVariablesValidator', () => {
     expect(act).to.not.throw();
   });
   describe('throws as expected', () => {
-    describe('"missing environment" if environment is not provided', () => {
-      itEachAbsentObjectValue((absentValue) => {
-        // arrange
-        const expectedError = 'missing environment';
-        const environment = absentValue;
-        // act
-        const act = () => validateEnvironmentVariables(environment);
-        // assert
-        expect(act).to.throw(expectedError);
-      });
-    });
     it('"missing keys" if environment has properties with missing values', () => {
       // arrange
       const expectedError = 'Environment keys missing: name, homepageUrl';