values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    BOOST_CHECK_EQUAL(
        HexStr(ParseHex_expected),
        "04678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5f");
    BOOST_CHECK_EQUAL(
        HexStr(Span<const unsigned char>(
               ParseHex_expected + sizeof(ParseHex_expected),
               ParseHex_expected + sizeof(ParseHex_expected))),
        "");
    BOOST_CHECK_EQUAL(
        HexStr(Span<const unsigned char>(ParseHex_expected, ParseHex_expected)),
        "");
    std::vector<unsigned char> ParseHex_vec(ParseHex_expected, ParseHex_expected + 5);
    BOOST_CHECK_EQUAL(
        HexStr(ParseHex_vec),
        "04678afdb0"
    );
}
BOOST_AUTO_TEST_CASE(util_Join)