Merge branch 'feature/skip-blank-xml' into 'master'

Issue warning and ignore empty XML tags

This MR modifies the session reader logic to ignore empty XML tags which would otherwise replace non-empty tags. This prevents, for example, the empty CONDITIONS section generated by NekMesh from displacing the user-provided conditions if the user condition file is supplied before the mesh file on the command-line.

@ssherw: This has been tested locally and everything passed. Just running through buildbot now.

See merge request !581