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/script/standard.h

@@ -20,7 +20,7 @@ class CScript;
 class CScriptID : public uint160
 {
 public:
-    CScriptID() : uint160(0) {}
+    CScriptID() : uint160() {}
     CScriptID(const CScript& in);
     CScriptID(const uint160& in) : uint160(in) {}
 };