HDDS-11045. Recon Decommissioning Info API throws NPE.

[devmadhuu]

What changes were proposed in this pull request?

This PR is to fix the NullPointerException for datanode tag in Decommissioning Info API JSON.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-11045

How was this patch tested?

Patch was tested manually with existing Junit test cases.

[devmadhuu]

@ArafatKhan2198 @sumitagrawl pls review.

[ArafatKhan2198]

Thanks for the patch @devmadhuu and @sumitagrawl for the review.