In 8d516ca9dd we tried to fix legacy
email search filters by allowing "platform/frameworks/base" to be
treated as a LabelType even though the string is not a valid name.
Unfortunately LabelType is super paranoid about checking its input
and the constructor refuses to accept this string.
Revert 8d516c and instead always match false if the label is not
defined in the project.
Change-Id: If1ab13bbf498df6cb8d02d868220653390045293
269649bda4