mprintf: remove strlen calls on empty strings in dprintf_formatf

Turns out that in dprintf_formatf we did a strlen on empty strings, a
bit strange is how common this actually is, 24 alone when doing a simple
GET from https://curl.se

Closes #8427

lib/mprintf.c

@@ -829,6 +829,8 @@ static int dprintf_formatf(
         }
         else if(prec != -1)
           len = (size_t)prec;
+        else if(*str == '\0')
+          len = 0;
         else
           len = strlen(str);