rpc/api: don't crash for unknown blocks

Most eth RPC calls that work with blocks crashed when the block was not
found because they called Hash on a nil block. This is a regression
introduced in cdc2662c (#1779).

While here, remove the insane conversions in get*CountBy*. There is no
need to construct a complete BlockRes and converting
int->int64->*big.Int->[]byte->hexnum->string to format the length of a
slice as hex.