getblockchaininfo: Fix case of pruning enabled but not pruned yet
getblockchaininfo: Fix case of pruning enabled but not pruned yet

This skips adding the 'pruneheight' entry when the blockchain is
marked as pruned but no blocks have been pruned yet. This might be a
state that is never reached.

(from clang static analysis)

BitcoinUnlimited/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited,Justaphf/BitcoinUnlimited,Justaphf/BitcoinUnlimited,BitcoinUnlimited/BitcoinUnlimited