Add groupService testing after the math issues; fix where needed

Underlying issue was groups not being removed/emptied when doing a new import.

src/main/java/nl/tudelft/ewi/queue/csv/CsvHelper.java

@@ -32,7 +32,7 @@ public class CsvHelper {
 
 	public static <T> List<T> readCsv(MultipartFile csvFile, Class<T> readerFor, CsvSchema schema)
 			throws IOException, EmptyCsvException, InvalidCsvException {
-		if (csvFile.isEmpty()) {
+		if (csvFile == null || csvFile.isEmpty()) {
 			throw new EmptyCsvException("Empty csv");
 		}