lookupInHash() implementations have logic error - path aliases not found

Problem/Motivation

Looking up sources for aliases sometimes results in "not found" even if the alias exists.

Sounds similar to #2625166: redis.path: Page not found for aliased path but isn't related to whitespaces - so here's a decicated ticket :)

The scenario is as follows:

1. Look up path for specific language e.g. contact-us-now (HGET prefix:path:s:en contact-us-now)
2. Redis_Path_PhpRedis::lookupInHash() will enable $doNoneLookup
3. Now if Redis_Path_PhpRedis::lookupInHash() doesn't find a language specific item it goes and tries to find a language neutral one (HGET prefix:path:s:und contact-us-now)
4. But if the language neutral item was explicitly set to not existent (Redis_Path_HashLookupInterface::VALUE_NULL = !) no return restore will happen due this code: if (!$ret && $previous) { - this again leads to returning FALSE which means "item not existent" even thought we actually just don't know if there's a language specific item.

Proposed resolution

I suggest to change the return restore condition to following:

          // If the language specific item was explicitly set to not-existent
          // and there's no language neutral item ensure the return is set to
          // not-existent.
          // OR
          // If the language neutral item is explicitly set to not-existent but
          // the language specific item is set to not found set the return to
          // not found - this will allow a lookup of the language specific item
          // and a proper cache set to not-existent if applicable.
          if ((!$ret && $previous) || (self::VALUE_NULL === $ret && empty($previous))) {
            // Restore null placeholder else we loose conversion to false
            // and drupal_lookup_path() would attempt saving it once again.
            $ret = $previous;
          }

As the added comment states. If the language neutral item is explicitly set to not-existent, but the language specific item just isn't found we shouldn't skip looking up the language specific item by returning FALSE but we should do a look up and ensure the language specific item is set to the appropriate value in the cache.
This will also avoid the language neutral lookup in sub-sequent requests which means it's a tiny bit more efficient (while not leading to inconsistencies e.g. due the language specific item being evicted when running redis as LRU).

Remaining tasks

Reviews needed.

User interface changes

None

API changes

Well the return of Redis_Path_PhpRedis::lookupInHash() / Redis_Path_Predis::lookupInHash() will change in some scenarios but that's fair I guess :)

Data model changes

None.