Fix DominantRC#compare() does not work as expected if cluster resource is empty

Steps to reproduce
=================
1. Install HA with 2 RM 2 NM (3072 MB * 2 total cluster)
2. Configure map and reduce size to 512 MB after changing scheduler minimum size to 512 MB
3. Configure capacity scheduler and AM limit to .5 (DominantResourceCalculator is configured)
4. Submit 30 concurrent task
5. Switch RM

Actual
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For 12 Jobs AM gets allocated and all 12 starts running
No other Yarn child is initiated , all 12 Jobs in Running state for ever

Expected
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Only 6 should be running at a time since max AM allocated is .5 (3072 MB)