Add empty party info response for older nodes to consume

[prd-fox]

Changes made for issue #649 and #833 mean that no party info is sent back as a response to the POST /partyinfo request. This is expected on older nodes and will cause them to error in that scenario.
Although in some scenarios they will continue to function with limited capability, in the case where the "key validation" feature is not enabled, there should be full compatibility between the nodes.

This change adds an empty response to these requests, so that older nodes can continue to function properly.
When the feature is enabled, it is required that all nodes are at least version 0.10.0, and so do not need any response entity.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@552b8b8). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #847   +/-   ##
=========================================
  Coverage          ?   99.44%           
  Complexity        ?     2076           
=========================================
  Files             ?      311           
  Lines             ?     6096           
  Branches          ?      312           
=========================================
  Hits              ?     6062           
  Misses            ?        5           
  Partials          ?       29

Impacted Files	Coverage Δ	Complexity Δ
...java/com/quorum/tessera/p2p/PartyInfoResource.java	100% <100%> (ø)	15 <0> (?)

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