ctf: loosen struct check to silently accept null fields

Change-Id: Ief637e1c6bc49aca4bed87b586224df9383c1a9e
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>

Author: MatthewKhouzam

ctf/org.eclipse.tracecompass.ctf.core/src/org/eclipse/tracecompass/internal/ctf/core/event/metadata/tsdl/struct/StructDeclarationParser.java

@@ -128,7 +128,8 @@ public StructDeclaration parse(ICTFMetadataNode declaration, ICommonTreeParserPa
         }
 
         scope.registerIdentifier(fieldName, decl);
-        if (struct.hasField(fieldName)) {
+        IDeclaration current = struct.getField(fieldName);
+        if (decl != null && current != null && !decl.equals(current)) {
             throw new ParseException("struct: duplicate field " //$NON-NLS-1$
                     + fieldName);
         }