graph.core: add comment to empty method

Add a nested comment explaining why this method
is empty, throw an UnsupportedOperationException
or complete the implementation.

Change-Id: Ic63615a8ccf01295e4b52f8c6434258464d4dcd8
Signed-off-by: Sehr Moosabhoy <sehr.moosabhoy@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/201771
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>

analysis/org.eclipse.tracecompass.analysis.graph.core/src/org/eclipse/tracecompass/internal/analysis/graph/core/graph/historytree/GraphHistoryTree.java

@@ -677,7 +677,7 @@ protected final void tryInsertAtNode(TmfEdgeInterval interval, int depth) {
      */
     @VisibleForTesting
     protected void informInsertingAtDepth(int depth) {
-
+        // do nothing
     }
 
     private long getNewBranchStart(int depth, TmfEdgeInterval interval) {