assertStringOrNull example

if (!is_int($options['contextLines']) || $options['contextLines'] < 0) {
            throw new ConfigurationException('contextLines', 'an int >= 0', $options['contextLines']);
        }

        if (!is_int($options['commonLineThreshold']) || $options['commonLineThreshold'] <= 0) {
            throw new ConfigurationException('commonLineThreshold', 'an int > 0', $options['commonLineThreshold']);
        }

        $this->assertString($options, 'fromFile');
        $this->assertString($options, 'toFile');
        $this->assertStringOrNull($options, 'fromFileDate');
        $this->assertStringOrNull($options, 'toFileDate');

        $this->header = sprintf(
            "--- %s%s\n+++ %s%s\n",
            $options['fromFile'],
            null === $options['fromFileDate'] ? '' : "\t" . $options['fromFileDate'],
            $options['toFile'],
            null === $options['toFileDate'] ? '' : "\t" . $options['toFileDate']
        );

        $this->collapseRanges      = $options['collapseRanges'];