Add more and unify tests for absent object cases

- Unify test data for nonexistence of an object/string and collection.
- Introduce more test through adding missing test data to existing tests.
- Improve logic for checking absence of values to match tests.
- Add missing tests for absent value validation.
- Update documentation to include shared test functionality.

tests/unit/shared/TestCases/AbsentTests.ts

@@ -0,0 +1,68 @@
+export function itEachAbsentStringValue(runner: (absentValue: string) => void): void {
+  itEachTestCase(AbsentStringTestCases, runner);
+}
+
+export function itEachAbsentObjectValue(runner: (absentValue: AbsentObjectType) => void): void {
+  itEachTestCase(AbsentObjectTestCases, runner);
+}
+
+export function itEachAbsentCollectionValue<T>(runner: (absentValue: []) => void): void {
+  itEachTestCase(getAbsentCollectionTestCases<T>(), runner);
+}
+
+function itEachTestCase<T>(
+  testCases: readonly IAbsentTestCase<T>[],
+  runner: (absentValue: T) => void,
+): void {
+  for (const testCase of testCases) {
+    it(`given "${testCase.valueName}"`, () => {
+      runner(testCase.absentValue);
+    });
+  }
+}
+
+export const AbsentObjectTestCases: readonly IAbsentTestCase<AbsentObjectType>[] = [
+  {
+    valueName: 'undefined',
+    absentValue: undefined,
+  },
+  {
+    valueName: 'null',
+    absentValue: null,
+  },
+];
+
+export const AbsentStringTestCases: readonly IAbsentStringCase[] = [
+  {
+    valueName: 'empty',
+    absentValue: '',
+  },
+  ...AbsentObjectTestCases,
+];
+
+export function getAbsentCollectionTestCases<T>(): readonly IAbsentCollectionCase<T>[] {
+  return [
+    ...AbsentObjectTestCases,
+    {
+      valueName: 'empty',
+      absentValue: new Array<T>(),
+    },
+  ];
+}
+
+type AbsentObjectType = undefined | null;
+
+interface IAbsentTestCase<T> {
+  valueName: string;
+  absentValue: T;
+}
+
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+interface IAbsentStringCase extends IAbsentTestCase<string> {
+  // Marker interface
+}
+
+// eslint-disable-next-line @typescript-eslint/no-empty-interface
+interface IAbsentCollectionCase<T> extends IAbsentTestCase<T[]> {
+  // Marker interface
+}