Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant)

src/qt/intro.cpp

@@ -226,7 +226,7 @@ bool Intro::showIfNeeded(bool& did_show_intro, int64_t& prune_MiB)
         }
 
         /* If current default data directory does not exist, let the user choose one */
-        Intro intro(0, Params().AssumedBlockchainSize(), Params().AssumedChainStateSize());
+        Intro intro(nullptr, Params().AssumedBlockchainSize(), Params().AssumedChainStateSize());
         intro.setDataDirectory(dataDir);
         intro.setWindowIcon(QIcon(":icons/bitcoin"));
         did_show_intro = true;