[avalanche] Fix ambiguous casts from size_t to UniValue
[avalanche] Fix ambiguous casts from size_t to UniValue

Summary: Introduced in D11696 and D11697

Test Plan:
  Build OSX

Reviewers: #bitcoin_abc, sdulfari, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D11720

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