Please make no-style selectable in match

Because the meta variable font style has no style value, it is not possible to write a match that affects meta but not other styles.

The following does not select the meta entry:

  <match target="scan">
    <test name="fullname">
      <string>Foo Italic</string>
    </test>
    <test name="family">
      <string>Foo</string>
    </test>
    <test name="style">
      <string></string>
    </test>
  </match>

The following does select the meta entry, but also all its Italic face

  <match target="scan">
    <test name="fullname">
      <string>Foo Italic</string>
    </test>
    <test name="family">
      <string>Foo</string>
    </test>
  </match>

This is wreaking havok in my fontconfig files.