RegexDecoder should not emit null record ids.

Fixes issue #23

Author: cboehme

src/main/java/org/culturegraph/mf/stream/converter/RegexDecoder.java

@@ -125,10 +125,12 @@ public void setDefaultLiteralName(final String defaultLiteralName) {
 	public void process(final String string) {
 		matcher.reset(string);
 
-		String id = null;
+		final String id;
 		final int groupIndex = captureGroupNames.indexOf(ID_CAPTURE_GROUP) + 1;
 		if (groupIndex > 0 && matcher.find()) {
 			id = matcher.group(groupIndex);
+		} else {
+			id = "";
 		}
 		getReceiver().startRecord(id);
 

src/test/java/org/culturegraph/mf/stream/converter/RegexDecoderTest.java

@@ -44,7 +44,7 @@ public final class RegexDecoderTest {
 	public void testRegex() {
 		final EventList expected = new EventList();
 
-		expected.startRecord(null);
+		expected.startRecord("");
 		expected.literal(DEFAULT_LITERAL_NAME, INPUT);
 		expected.literal(GROUP_NAME_1, "42");
 		expected.literal(GROUP_NAME_2, "xyzzy");