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/miner.cpp

@@ -103,7 +103,7 @@ void UpdateTime(CBlockHeader* pblock, const Consensus::Params& consensusParams,
     pblock->nTime = std::max(pindexPrev->GetMedianTimePast()+1, GetAdjustedTime());
 
     // Updating time can change work required on testnet:
-    if (consensusParams.nPowAllowMinDifficultyBlocksAfterHeight) {
+    if (consensusParams.nPowAllowMinDifficultyBlocksAfterHeight != boost::none) {
         pblock->nBits = GetNextWorkRequired(pindexPrev, pblock, consensusParams);
     }
 }
@@ -727,7 +727,7 @@ void static BitcoinMiner()
                 // Update nNonce and nTime
                 pblock->nNonce = ArithToUint256(UintToArith256(pblock->nNonce) + 1);
                 UpdateTime(pblock, chainparams.GetConsensus(), pindexPrev);
-                if (chainparams.GetConsensus().nPowAllowMinDifficultyBlocksAfterHeight)
+                if (chainparams.GetConsensus().nPowAllowMinDifficultyBlocksAfterHeight != boost::none)
                 {
                     // Changing pblock->nTime can change work required on testnet:
                     hashTarget.SetCompact(pblock->nBits);

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: