DefaultConverter: fix null primitive values bug

If the destination type is a primitive, we cannot return null.
We must return the designated null-like value for that type instead.

Author: ctrueden

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

@@ -185,7 +185,7 @@ public <T> T convert(final Object src, final Class<T> dest) {
 		catch (final Exception exc) {
 			// TODO: Best not to catch blanket Exceptions here.
 			// no known way to convert
-			return null;
+			return Types.nullValue(dest);
 		}
 	}