Replace direct use of 0 with SetNull and IsNull

Replace x=0 with .SetNull(),
x==0 with IsNull(), x!=0 with !IsNull().
Replace uses of uint256(0) with uint256().

src/primitives/block.h

@@ -53,8 +53,8 @@ public:
     void SetNull()
     {
         nVersion = CBlockHeader::CURRENT_VERSION;
-        hashPrevBlock = 0;
-        hashMerkleRoot = 0;
+        hashPrevBlock.SetNull();
+        hashMerkleRoot.SetNull();
         nTime = 0;
         nBits = 0;
         nNonce = 0;