Config: Prevent access to size of non-existent read-data/write-data

[MakisH]

Bugfix for an issue reported by a user in https://precice.discourse.group/t/error-from-the-calculix-adapter-for-cht-coupling-with-heat-flux/2718

Running the flow-over-heated-plate-two-meshes tutorial, the CalculiX adapter fails with:

terminate called after throwing an instance of ‘fkyaml::v0_4_2::type_error’
what():  type_error: The target node is not of a container type. type=NULL_OBJECT

because the configuration only includes one of write-data/read-data per interface, but not both:

participants:
    Solid:
        interfaces:
        - nodes-mesh: Solid-Mesh-nodes
          patch: yend
          read-data: [Temperature]
        - faces-mesh: Solid-Mesh
          patch: flux_interface
          write-data: [Heat-Flux]

triggering a behavior similar to the one described in #35 for uni-directional coupling per interface.

This PR moves the config["participants"][participantName]["interfaces"][i]["write-data"].size(); operator into the branch that first checks if config["participants"][participantName]["interfaces"][i]["write-data"] exists.

No changelog entry is needed, since this has not been released yet (introduced after the latest release, in #143).

Merging to ship it to the user. Changes are rather trivial, and I considered documentation.

System test is following.

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/error-from-the-calculix-adapter-for-cht-coupling-with-heat-flux/2718/4