Fix NullConverter for null sources

We want the NullConverter to explicitly support null sources (and
in fact, only null sources), so it must override all the canConvert
method implementations accordingly.

Author: ctrueden

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

@@ -31,6 +31,8 @@
 
 package org.scijava.convert;
 
+import java.lang.reflect.Type;
+
 import org.scijava.Priority;
 import org.scijava.plugin.Plugin;
 import org.scijava.util.ConversionUtils;
@@ -54,9 +56,26 @@
 @Plugin(type = Converter.class, priority = Priority.FIRST_PRIORITY)
 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;
+	}
+
+	@Override
+	public boolean canConvert(final Object src, final Type dest) {
+		return src == null && dest != null;
+	}
+
 	@Override
 	public boolean canConvert(final Object src, final Class<?> dest) {
-		return src == null;
+		return src == null && dest != null;
+	}
+
+	@Override
+	public boolean canConvert(final Class<?> src, final Class<?> dest) {
+		return src == null && dest != null;
 	}
 
 	@Override
@@ -78,11 +97,4 @@ public Class<Object> getInputType() {
 		return Object.class;
 	}
 
-	// -- Deprecated API --
-
-	@Override
-	@Deprecated
-	public boolean canConvert(final Class<?> src, final Class<?> dest) {
-		return false;
-	}
 }