[6.x] Prevent deleted entries from being restored

[duncanmcclean]

This pull request fixes an issue where a long-lived worker could recreate a deleted entry as an empty stub file.

This was happening because a stale in-memory Stache path could outlive the shared cached entry. A cache miss then attempted to parse the removed file, which generated and wrote a new entry.

This PR fixes it by returning no item when the path no longer exists.

Fixes #15225

[jasonvarga]

Warning: Value::getItems() (src/Stache/Indexes/Value.php:11) doesn't filter out the null items BasicStore::getItem() can now return. This index class is the default value index for BasicStore-based stores, used whenever a query filters/sorts on a field whose index isn't cached yet — not a rare path.

Trace: getItemsFromFiles() → getItemValue(null) → (new ResolveValue)(null, $name). Once $name doesn't start with data->, the loop's getItemPartValue(null, $part) falls through every check (is_array, is_scalar, ContainsQueryableValues, method_exists) to return $item->get($name); with $item === null — a fatal error.

Before this PR, the same stale-path race produced a corrupt-but-non-null stub Entry here, so ResolveValue never crashed — just resolved to wrong/empty data. After this PR, the exact race this PR fixes can now hard-crash index building instead of degrading silently. Query\Builder::getItems() (src/Stache/Query/Builder.php:146) already does ->filter()->values() and is unaffected — Value::getItems() is the one unguarded consumer of getItemsFromFiles().

Suggested fix: $this->store->getItemsFromFiles()->filter()->map(...).