Process a single-line array.

While the WP standard does not allow single line multi-item associative arrays,
this sniff should function independently of that.

The `WordPress.WhiteSpace.OperatorSpacing` sniff already covers checking that
there is a space between the array key and the double arrow, but doesn't
enforce it to be exactly one space for single line arrays.
That is what this method covers.

@since 0.14.0

@param int   $stackPtr The position of the current token in the stack.
@param array $items    Info array containing information on each array item.
@param int   $opener   The position of the array opener.
@param int   $closer   The position of the array closer.

@return int|void Integer stack pointer to skip forward or void to continue
normal file processing.