Allow Query::getFields() to return NULL

Thanks a lot for creating the patch, it's definitely a good start!

However, there's some problems with it currently:

1. +++ b/src/Entity/Index.php
   @@ -582,6 +582,11 @@ class Index extends ConfigEntityBase implements IndexInterface {
   +    // Add fulltext fields, unless set.
   +    if ($this->fields === NULL) {
   +      $this->fields = $this->index->getFulltextFields();
   +    }
   

   Any particular reason you chose the second way of fixing this?
   As said, I think I'd have preferred the former one (allowing a NULL return value).

   Oh, also this is in the completely wrong method. It should, if at all, be in the query class's getFulltextFields() method, not in the index class. (And should then be removed from the query class's preExecute() method.)

2. +++ b/src/Plugin/views/query/SearchApiQuery.php
   @@ -566,12 +566,11 @@ class SearchApiQuery extends QueryPluginBase {
   -  // @todo Allow calling with NULL, and maybe rename to setFulltextFields().
   -  public function fields(array $fields) {
   +  public function setFulltextFields($fields = array()) {
   

   array $fields = NULL is the argument list I had in mind. (Also in the two other method definitions.)

Great work, thanks! Looks pretty good now already. Some more nit-picking, though:

1. +++ b/src/Plugin/views/query/SearchApiQuery.php
   @@ -566,12 +566,11 @@ class SearchApiQuery extends QueryPluginBase {
   -  public function fields(array $fields) {
   +  public function setFulltextFields($fields = NULL) {
   

   This can still use the array type hint if the default value is NULL – a perhaps little-known but very handy feature of PHP (same for class/interface type hints).

2. +++ b/src/Query/QueryInterface.php
   @@ -231,9 +230,9 @@ interface QueryInterface {
   -   * @return string[]
   +   * @return NULL|string[]
       *   An array containing the fields that should be searched for the search
   -   *   keys.
   +   *   keys or null.
   

   Exactly the other way round – "null" next to the @return tag but then "NULL" in the description.

   Also, the description should say in which situation NULL is returned. And I'd list the return types as string[]|null, since that's the "main" type. (I usually put "null" last in such lists – but of course purely a matter of taste.)

Also, now that NULL is an allowed return value for getFulltextFields(), we don't have to explicitly set them to anything in Query::preProcess() – so, please remove that code, too.

Otherwise, this is fine, I can probably commit it once these are fixed.
Thanks again!

Yes, when we change the contract for getFields(), we of course also need to change the backend plugin accordingly.
(Interdiff based on #9.)

Committed.
Thanks again for your great work on this!