AbstractConverter: can't convert null inputs

The default converter contract is that inputs can not be null, with the
intent that individual converters can otherwise override this behavior.

This contract is now consistent between all canConvert signatures, and
has been clarified in the javadoc.

See imagej/imagej-common#52

Author: hinerm

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

@@ -55,6 +55,14 @@
  * implementation would like to suggest candidates for conversion, this method
  * can be overridden.
  * </p>
+ * <p>
+ * NB: by default, the provied {@link #canConvert} methods will return
+ * {@code false} if the input is {@code null}. This allows {@link Converter}
+ * implementors to assume any input is non-{@code null} - but this behavior is
+ * overridden. Casting {@code null Object} inputs is handled by the
+ * {@link NullConverter}, while {@code null class} inputs are handled by the
+ * {@link DefaultConverter}.
+ * </p>
  *
  * @author Mark Hiner
  */
@@ -99,6 +107,7 @@ public boolean canConvert(final Object src, final Class<?> dest) {
 
 	@Override
 	public boolean canConvert(final Class<?> src, final Class<?> dest) {
+		if (src == null) return false;
 		return ConversionUtils.canCast(src, getInputType()) &&
 			ConversionUtils.canCast(getOutputType(), dest);
 	}