Merge bitcoin/bitcoin#33566: miner: fix empty mempool case for waitNext()

8f7673257a miner: fix empty mempool case for waitNext() (Sjors Provoost)

Pull request description:

  Block template fees are calculated by looping over `new_tmpl->vTxFees` and return (early) once the `fee_threshold` is exceeded.

  This left an edge case when the mempool is empty, which this commit fixes and adds a test for.

  Also update `test/functional/interface_ipc.py` to reflect the new behavior,

  Fixes https://github.com/Sjors/sv2-tp/issues/9

ACKs for top commit:
  optout21:
    ACK 8f7673257a
  cedwies:
    tACK 8f76732
  sipa:
    utACK 8f7673257a
  zaidmstrr:
    Concept ACK [8f76732](8f7673257a)

Tree-SHA512: ef200fe95e96f810e425283bc37f945c4bf5efa16f4b74820b8a07968f30c5146bca213a372124be84b48beead5dfd35f2b5d10d188d0a465f847ebab61de10a

test/functional/interface_ipc.py

@@ -153,6 +153,7 @@ class IPCInterfaceTest(BitcoinTestFramework):
             self.log.debug("Wait for a new template")
             waitoptions = self.capnp_modules['mining'].BlockWaitOptions()
             waitoptions.timeout = timeout
+            waitoptions.feeThreshold = 1
             waitnext = template.result.waitNext(ctx, waitoptions)
             self.generate(self.nodes[0], 1)
             template2 = await waitnext
@@ -168,6 +169,7 @@ class IPCInterfaceTest(BitcoinTestFramework):
             block3 = await self.parse_and_deserialize_block(template4, ctx)
             assert_equal(len(block3.vtx), 2)
             self.log.debug("Wait again, this should return the same template, since the fee threshold is zero")
+            waitoptions.feeThreshold = 0
             template5 = await template4.result.waitNext(ctx, waitoptions)
             block4 = await self.parse_and_deserialize_block(template5, ctx)
             assert_equal(len(block4.vtx), 2)