Don't set wxTextAttr font family to invalid value.

Don't set wxTextAttr font family to invalid value.

wxTextAttr::HasFontFamily() shouldn't return true if there is no valid font
family in this attribute but this could happen if it was constructed from a
font which didn't know its own family.

This fixes asserts on the startup of the text sample in wxMSW due to passing
wxFONTFAMILY_UNKNOWN to wxFont::SetFamily() when trying to use such invalid
attribute later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62720 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775