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<!-- Describe the issue -->
When using listsinceblock with a target_confirmations greater than the current block count, it returns "0000000000000000000000000000000000000000000000000000000000000000" as lastblock, which makes the method fail when using that value as blockhash
Expected behavior
<!--- What behavior did you expect? -->
The method shouldn't throw an error for a return value that is supposed "to feed back into listsinceblock the next time you call it"
Actual behavior
<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->
RPC returns error code -5, "Block not found"
To reproduce On a pristine node where "blocks" is currently at 0, call listsinceblock '' 6, it will return "0000000000000000000000000000000000000000000000000000000000000000" as lastblock, and on next call (listsinceblock "0000000000000000000000000000000000000000000000000000000000000000" 6 it will throw an error
<!--- How reliably can you reproduce the issue, what are the steps to do so? -->
System information
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It can be reliable reproducible without any extra steps
Proposed solution Returning the genesis block hash when target_confirmations exceeds the current block count
Note: This is particularly useful on regtest, as on mainnet you would need to call with "640645" (as of now) to get the described issue, but on a pristine regtest even using a value as low as 2 makes the returned value unsuitable for next calls.
