mbmd: No output for model sunspec 213

With PR #191 support for meter float model sunspec 213 was introduced. I have a Fronius Smart Meter 63A which reports as such a model.

With mbmd inspect, I get all measurement correctly:

> docker run --rm -p 8080:8080 volkszaehler/mbmd  inspect -d SUNS:240@symo:502
2021/04/16 21:51:34 config: creating TCP connection for symo:502
--------- Model 1 common ---------
Mn            Fronius    string
Md    Smart Meter 63A    string
Opt          3.16.7-1    string
Vr                2.9    string
SN           18410005    string
DA             240.00    uint16
Pad            0x00d5    pad
--------- Model 213 ac_meter ---------
A                            3.05    float32
AphA                        0.929    float32
AphB                         1.48    float32
AphC                        0.641    float32
PhV                     244.23334    float32
PhVphA                      243.7    float32
PhVphB                      243.7    float32
PhVphC                      245.3    float32
PPV                     423.03333    float32
...

With mbmd run, connection is established, the device is again detected correctly, but no values are reported:

> docker run --rm -v $(pwd)/mbmd.yml:/etc/mbmd.yaml -p 8080:8080 volkszaehler/mbmd run -v
2021/04/16 22:17:33 mbmd 0.13 (fa6b574)
2021/04/16 22:17:33 config: using /etc/mbmd.yaml
2021/04/16 22:17:33 config: creating TCP connection for symo:502
2021/04/16 22:17:33 config: declared device SUNSPEC:240.0
2021/04/16 22:17:33 httpd: starting api at 0.0.0.0:8080
2021/04/16 22:17:35 initialized device SUNSPEC1.240: {SUNSPEC Fronius Smart Meter 63A 3.16.7-1 2.9 18410005 0}
^C2021/04/16 22:21:45 received signal - stopping
2021/04/16 22:21:45 stopped

Output stops until interrupted with CTRL-C.