Fix HLOOKUP on single row

[arnested]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Fixes a bug when doing a HLOOKUP on a single row.

<?php

require 'vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\Spreadsheet;

$spreadsheet = new Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();

/**
 * Single row.
 */
$singleRow = "=HLOOKUP(10, {5, 10, 15}, 1, 0)";
$sheet->getCell('A1')->setValue($singleRow);

// Should echo 10, but echos '#N/A' and some PHP notices and warnings.
echo $sheet->getCell('A1')->getCalculatedValue() . PHP_EOL;

/**
 * Multiple rows.
 */
$multipleRows = "=HLOOKUP(10, {5, 10, 15; 20, 25, 30}, 1, 0)";
$sheet->getCell('A2')->setValue($multipleRows);

// Should echo: 10 and also does.
echo $sheet->getCell('A2')->getCalculatedValue() . PHP_EOL;

Before fix:

PHP Notice:  Undefined offset: 0 in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 815

Notice: Undefined offset: 0 in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 815
PHP Notice:  Undefined index:  in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 819

Notice: Undefined index:  in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 819
PHP Warning:  Invalid argument supplied for foreach() in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 819

Warning: Invalid argument supplied for foreach() in /Users/arne/PhpSpreadsheet/src/PhpSpreadsheet/Calculation/LookupRef.php on line 819
#N/A
10

After fix:

10
10

[MarkBaker]

Thank you for identifying and fixing a case that we hadn't considered when doing the implementation of HLOOKUP()