Fix column types wrongly set to TYPE_STRING in JSONColumns

when the column contains NULL values.
  modified:   storage/connect/tabjson.cpp

MDEV-12651: change error code to ER_ILLEGAL_HA in rnd_pos.
  modified:   storage/connect/ha_connect.cc

storage/connect/ha_connect.cc

@@ -4009,7 +4009,7 @@ int ha_connect::rnd_pos(uchar *buf, uchar *pos)
 	} else {
 		PGLOBAL g = GetPlug((table) ? table->in_use : NULL, xp);
 		strcpy(g->Message, "Not supported by this table type");
-		my_message(ER_UNKNOWN_ERROR, g->Message, MYF(0));
+		my_message(ER_ILLEGAL_HA, g->Message, MYF(0));
 		rc= HA_ERR_INTERNAL_ERROR;
 	}	// endif SetRecpos
 

storage/connect/tabjson.cpp

@@ -288,8 +288,13 @@ PQRYRES JSONColumns(PGLOBAL g, char *db, char *dsn, PTOS topt, bool info)
           break;
   
       if (jcp) {
-        if (jcp->Type != jcol.Type)
-          jcp->Type = TYPE_STRING;
+				if (jcp->Type != jcol.Type) {
+					if (jcp->Type == TYPE_UNKNOWN)
+						jcp->Type = jcol.Type;
+					else if (jcol.Type != TYPE_UNKNOWN)
+					  jcp->Type = TYPE_STRING;
+
+				} // endif Type
 
         if (*fmt && (!jcp->Fmt || strlen(jcp->Fmt) < strlen(fmt))) {
           jcp->Fmt = PlugDup(g, fmt);