SeriesInfoExtractorServiceImpl.extractQuantity(): fix NPE.

Correction for 6ea4c8bf66e385ad92ce93d9ecb820809326cef4 commit.

Addressed to #781

Author: php-coder

src/main/java/ru/mystamps/web/service/SeriesInfoExtractorServiceImpl.java

@@ -153,6 +153,10 @@ public Integer extractReleaseYear(String fragment) {
 	// @todo #781 SeriesInfoExtractorServiceImpl.extractQuantity() respect MAX_STAMPS_IN_SERIES
 	@Override
 	public Integer extractQuantity(String fragment) {
+		if (StringUtils.isBlank(fragment)) {
+			return null;
+		}
+		
 		Matcher matcher = NUMBER_OF_STAMPS_REGEXP.matcher(fragment);
 		if (matcher.find()) {
 			return Integer.valueOf(matcher.group(1));