casa-mp-base

perl5: Do not add conflicts for a single variant

Previously, the perl5 port group would produce lint warnings if only
a single variant was generated, because it added conflicts with an empty
string. Do not add the conflicts argument if there is no other variant.

    $ port lint amavisd-new
    --->  Verifying Portfile for amavisd-new
    Warning: Variant perl5_24 conflicts with non-existing variant
    --->  0 errors and 1 warnings found.