Hi,

Empty macros does not seem to work in 3.x, it pass "{}" to the command line

These commands work

```
zabbix_get -s my_computer.local -k 'system.run["/usr/local/sbin/check_mongodb.py -u \"\" -p \"\" -A connect nd -M 3 2>&1"]'
```
or
```
zabbix_get -s my_computer.local -k 'system.run["/usr/local/sbin/check_mongodb.py -u -p -A connect nd -M 3 2>&1"]'
```

But this item
```
system.run["/usr/local/sbin/check_mongodb.py -u {$MONGO_ADMIN} -p {$MONGO_PASSWORD} -A connect nd -M {$MONGO_VERSION} 2>&1"]
```

Isn't working when macro values are set to:
```
{$MONGO_PASSWORD} =>
{$MONGO_USER} =>
{$MONGO_VERSION} => 3
```

```check_mongodb.py: error: option -u: invalid choice: '{}' ```

I've also tried to put double double quotes in the macro value, but not better
```
CRITICAL - General MongoDB Error: Authentication failed.
```