Fix regression on /names/user returning empty list

[vangberg]

Fixes #303

NamesController#user sets params[:status] = 'all'. Before pull request #288 this was converted into opts[:status] = nil:

    opts[:status] ||=
      case @status.to_s.downcase
      when 'public';    Name.public_status
      when 'automated'; 0
      when 'seqcode';   15
      when 'icnp';      20
      when 'icnafp';    25
      when 'valid';     Name.valid_status
      end

The pull request changed that code to call map_status_to_value, which returns 'all'. This in turn would be sent to apply_name_filters which would create the condition WHERE status = 'all', returning nothing. By explicitly returning nil instead, no conditions on status are imposed.