Ensure that the sqlite3StrAccumAppend() routine is never called with a NULL

second argument.  Doing so is harmless when N==0, but it causes an assert()
to fail that was placed to quiet static analyzers.

FossilOrigin-Name: f03fbf3700d9d4a654e3aa2e5caa810a8416bed9

src/func.c

@@ -1541,7 +1541,9 @@ static void groupConcatStep(
     }
     zVal = (char*)sqlite3_value_text(argv[0]);
     nVal = sqlite3_value_bytes(argv[0]);
-    if( nVal || firstTerm ) sqlite3StrAccumAppend(pAccum, zVal, nVal);
+    if( nVal || (zVal="", firstTerm) ){
+      sqlite3StrAccumAppend(pAccum, zVal, nVal);
+    }
   }
 }
 static void groupConcatFinalize(sqlite3_context *context){