Fix NullConverter to match for both null src and dest

Before the NullConverter was only matched for null src Objects. This
converter still will not be matched if canConvert(Class<?>, Class<?>) is
called with a null source Class. Additionally, the ConverterTest has
been updated to reflect this change.

Author: awalter17

src/main/java/org/scijava/convert/NullConverter.java

@@ -60,23 +60,24 @@ public class NullConverter extends AbstractConverter<Object, Object> {
 	@Override
 	public boolean canConvert(final ConversionRequest request) {
 		if (request == null) return false;
-		if (request.destType() == null && request.destClass() == null) return false;
-		return request.sourceObject() == null && request.sourceClass() == null;
+		return (request.destType() == null && request.destClass() == null) ||
+			(request.sourceObject() == null && request.sourceClass() == null);
 	}
 
 	@Override
 	public boolean canConvert(final Object src, final Type dest) {
-		return src == null && dest != null;
+		return src == null || dest == null;
 	}
 
 	@Override
 	public boolean canConvert(final Object src, final Class<?> dest) {
-		return src == null && dest != null;
+		return src == null || dest == null;
 	}
 
 	@Override
 	public boolean canConvert(final Class<?> src, final Class<?> dest) {
-		return src == null && dest != null;
+		if (src == null) return false;
+		return dest == null;
 	}
 
 	@Override

src/test/java/org/scijava/convert/ConverterTest.java

@@ -67,7 +67,7 @@ public void testNullConverter() {
 		final NullConverter nc = new NullConverter();
 		assertFalse(nc.canConvert(Object.class, Object.class));
 		assertFalse(nc.canConvert(Object.class, (Type) Object.class));
-		assertTrue(nc.canConvert((Class<?>) null, Object.class));
+		assertFalse(nc.canConvert((Class<?>) null, Object.class));
 		assertTrue(nc.canConvert((Object) null, Object.class));
 		assertTrue(nc.canConvert((ConverterTest) null, ArrayList.class));
 		assertNull(nc.convert((Object) null, Object.class));