fix db_mysql getRow() when column is null error raised (#12806) [backport]

* fix db_mysql getRow()
* added if y == nil [backport]

(cherry picked from commit 208b018f6b)

lib/system.nim

@@ -3423,9 +3423,10 @@ elif hasAlloc:
   {.push stack_trace:off, profiler:off.}
   proc add*(x: var string, y: cstring) =
     var i = 0
-    while y[i] != '\0':
-      add(x, y[i])
-      inc(i)
+    if y != nil:
+      while y[i] != '\0':
+        add(x, y[i])
+        inc(i)
   {.pop.}
 
 when defined(nimvarargstyped):