feature: support empty block range end in getaddresstxids calls

[conradoplg]

Motivation

The end argument in the getaddresstxids RPC is mandatory in Zebra, because it was always being specified in lightwalletd and supporting lightwalletd was our goal at the time. However, it will be changed to possibly not specify end, so we need to make it optional if we want to keep supporting lightwalletd.

Specifications

https://zcash-rpc.github.io/getaddresstxids.html

It's not specified, but if end is missing, the current best chain tip height should be used.

Complex Code or Requirements

No response

Testing

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Related Work

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