doclets/gtkdoc: Do not set section comment from a namespace with empty doc
doclets/gtkdoc: Do not set section comment from a namespace with empty doc

Fixes bug 648985.

GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala