[9.x] Add soleValue method to query builders

[hailwood]

Currently we have the following methods

$query->sole(['id']); // bails if there is not exactly one result, but returns an object
$query->value('id');  // returns just the value, but no safety around there being exactly one result

However if we want the id of the sole result we must do $query->sole(['id'])->id.

This PR adds a new method to both builders soleValue which is the same logic as value (removing the null checks since sole will bail) and uses sole instead of first internally allowing us to do

$query->sole(['id']);    // bails if there is not exactly one result, but returns an object
$query->value('id');     // returns just the value, but no safety around there being exactly one result
$query->soleValue('id'); // bails if there is not exactly one result and returns just the value