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.

src/presentation/components/Scripts/View/ScriptsTree/SelectableTree/SelectableTree.vue

@@ -67,7 +67,7 @@ export default class SelectableTree extends Vue { // Stateless to make it easier
   @Watch('initialNodes', { immediate: true })
   public async updateNodes(nodes: readonly INode[]) {
     if (!nodes) {
-      throw new Error('undefined initial nodes');
+      throw new Error('missing initial nodes');
     }
     const initialNodes = nodes.map((node) => toNewLiquorTreeNode(node));
     if (this.selectedNodeIds) {