Add guard for empty preg_match_all in processVariableInString #272

Job	Run time
Test: PHP 5.5 on PHPCS 3.5.6	32s
Test: PHP 5.5 on PHPCS dev-master	31s
Test: PHP 5.6 on PHPCS 3.5.6	26s
Test: PHP 5.6 on PHPCS dev-master	23s
Test: PHP 7.0 on PHPCS 3.5.6	14s
Test: PHP 7.0 on PHPCS dev-master	37s
Test: PHP 7.1 on PHPCS 3.5.6	19s
Test: PHP 7.1 on PHPCS dev-master	19s
Test: PHP 7.2 on PHPCS 3.5.6	23s
Test: PHP 7.2 on PHPCS dev-master	22s
Test: PHP 7.3 on PHPCS 3.5.6	14s
Test: PHP 7.3 on PHPCS dev-master	26s
Test: PHP 8.2 on PHPCS 3.6.1	19s
Test: PHP 8.1 on PHPCS dev-master	19s
Test: PHP 8.1 on PHPCS 3.6.1	13s
Test: PHP 8.0 on PHPCS dev-master	27s
Test: PHP 8.0 on PHPCS 3.5.7	14s
Test: PHP 7.4 on PHPCS dev-master	20s
Test: PHP 8.3 on PHPCS dev-master	1m 3s
Coverage: PHP 8.2 on PHPCS dev-master	22s
Coverage: PHP 7.4 on PHPCS 3.5.6	29s
Coverage: PHP 5.4 on PHPCS dev-master	1m 29s
Coverage: PHP 5.4 on PHPCS 3.5.6	1m 44s
coveralls-finish	4s
	11m 49s

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