Avoid converting null values

In setValue calls we want to minimize the usage of the ConvertService,
so if if the given value is null we should just cast.

Author: hinerm

src/main/java/org/scijava/util/ClassUtils.java

@@ -431,7 +431,7 @@ public static void setValue(final Field field, final Object instance,
 		try {
 			field.setAccessible(true);
 			final Object compatibleValue;
-			if (field.getType().isInstance(value)) {
+			if (value == null || field.getType().isInstance(value)) {
 				// the given value is compatible with the field
 				compatibleValue = value;
 			}