Loads a string source object, fast query.
These 'fast query' methods are the ones in the critical path and their implementation must be optimized for speed, as they may run many times in a single page request.
array $conditions: (optional) Array with conditions that will be used to filter the strings returned and may include all of the conditions defined by getStrings().
\Drupal\locale\SourceString|null Minimal TranslationString object if found, NULL otherwise.
public function findString(array $conditions);