fix the printout of empty strings in the yaml cards of the catalog

[dafnapension]

Fix, e.g, from:

to:

[dafnapension]

And removed some red lines from make servers-doc

A few missing ref targets remained for @elronbandel to resolve:

/home/.../unitxt/docs/docs/examples.rst:154: WARNING: undefined label: 'creating a custom criteria'
/home/.../unitxt/docs/docs/examples.rst:165: WARNING: undefined label: 'end to end direct example'
/home/.../unitxt/docs/docs/examples.rst:189: WARNING: undefined label: 'end to end pairwise example'