Tables: actually use null for null column headers

If the map has a null key, use null for that column header. The Table
API does allow for null column headers (which means no header), so
there is no need to label that header with a string like "<null>".

Author: ctrueden

src/main/java/org/scijava/table/Tables.java

@@ -97,7 +97,7 @@ public String getHeader() {
 							int c = 0;
 							for (final Object key : data.get(0).keySet()) {
 								if (col == c++) {
-									colHeader = key == null ? "<null>" : key.toString();
+									colHeader = key == null ? null : key.toString();
 									break;
 								}
 							}