Explicitly check the min-difficulty flag against boost::none

It isn't clear how a boost::optional that holds 0 (which is the case for
regtest) is coerced to a boolean, unless you pore over the Boost
documentation. An explicit check is clearer.

src/pow.cpp

@@ -27,7 +27,7 @@ unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHead
     {
         // Comparing to pindexLast->nHeight with >= because this function
         // returns the work required for the block after pindexLast.
-        if (params.nPowAllowMinDifficultyBlocksAfterHeight &&
+        if (params.nPowAllowMinDifficultyBlocksAfterHeight != boost::none &&
             pindexLast->nHeight >= params.nPowAllowMinDifficultyBlocksAfterHeight.get())
         {
             // Special difficulty rule for testnet: