[SlurmTopo] Solve the update failures by defining when to update topology.conf

[himani2411]

Description of changes

• Solve the update failures by defining when to update topology.conf only if p6egb200_block_sizes is not nil.
• Add a default value of p6egb200_block_sizes as nil

Tests

• Unit Test
• Successfully created a cluster and force updated the cluster.

DevSettings:
 Cookbook:
   ChefCookbook: https://github.com/himani2411/aws-parallelcluster-cookbook/tarball/slurm-topo-update-fix

References

• Link to impacted open issues.
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