Disallow empty glyph name decoded from post table

But preserve it...

For example, with this font:

https://code.google.com/p/googlefontdirectory/source/browse/ofl/phetsarath/Phetsarath-Regular.ttf

we now get:

    ...
    <GlyphID id="136" name="uni0EDD.am"/>
    <GlyphID id="137" name="uni0EB3.right"/>
    <GlyphID id="138" name="glyph00138"/>
    <GlyphID id="139" name="glyph00139"/>
    <GlyphID id="140" name="glyph00140"/>
    ...
    <GlyphID id="161" name="glyph00161"/>
  </GlyphOrder>

...

  <post>
    <formatType value="2.0"/>
    <italicAngle value="0.0"/>
    <underlinePosition value="-217"/>
    <underlineThickness value="150"/>
    <isFixedPitch value="0"/>
    <minMemType42 value="0"/>
    <maxMemType42 value="0"/>
    <minMemType1 value="0"/>
    <maxMemType1 value="0"/>
    <psNames>
      <!-- This file uses unique glyph names based on the information
           found in the 'post' table. Since these names might not be unique,
           we have to invent artificial names in case of clashes. In order to
           be able to retain the original information, we need a name to
           ps name mapping for those cases where they differ. That's what
           you see below.
            -->
      <psName name="glyph00107" psName=""/>
      <psName name="glyph00138" psName=""/>
      <psName name="glyph00139" psName=""/>
      ...
      <psName name="glyph00161" psName=""/>
    </psNames>

Part of https://github.com/behdad/fonttools/issues/124
Ignore "reserved" post table format 2.0 indices

Fixes https://github.com/behdad/fonttools/issues/124