Passing NULL or "" as $group when adding Views conditions is deprecated

The SearchApiQuery methods addConditionGroup() and addCondition() currently let their $group parameter default to NULL. Both this and "" will then be converted to 0 in the method body. (This mirrors the behavior of the addWhere() and similar methods in the Sql query class provided by Views itself.)

Since this makes the code more confusing, as NULL is presented as the default value but will in fact never be used as a group ID, the default was now switched to an explicit 0 (with the exact same meaning) and passing anything other than an integer or an integer-compatible string is deprecated.

However, as $group is the last parameter for both methods, it will usually be omitted by callers when conditions should be added to the default group, so actual changes to the code will rarely be necessary.

Before:

$views_query->addCondition('type', 'article', '=', NULL);

or

$views_query->addCondition('type', 'article', '=', '');

After:

$views_query->addCondition('type', 'article', '=', 0);

or

$views_query->addCondition('type', 'article', '=');