Fix/permission and getmaps ys

[PerrineAguiar]

This PR fixes two issues found while testing on YS:

• start the tango nodelet only after asking for tango permissions
• returning an empty string if there is no map on the device yet.

[PerrineAguiar]

@mcopejans Thanks for the review, I addressed your comments.

[mcopejans]

OK, the implementation is quite conservative because permissions for both adf and dataset are always required even if not needed. This could be addressed as a follow up? LGTM

[PerrineAguiar]

@mcopejans Both permissions are not required to be allowed, but to be asked, i.e. we wait for the user to answer both permissions pop up before starting the node. I will add some comments in the code so things are clearer.

[mcopejans]

Asking would also not be required in all cases. Only if specific settings are enabled.

[PerrineAguiar]

Yes, I'll make a follow up to address this.