Don't throw an error when namespace indexers don't find a valid key

Don't throw an error when namespace indexers don't find a valid key

Examples:

```
globals["abc"]
globals.def
```

The patch for the Icinga Director unfortunately only solves the
master, and as discussed with @lippserd we need to ensure that
satellites and clients with 2.10 can be restarted without any errors
from deployed configuration.

refs #6509
refs icinga/icingaweb2-module-director#1654