maint - check if there are results to sparkline first

Array#min and Array#max on an empty array return nil.  Here we check
if the array if empty first.

Avoids this segfault.

    mco puppet summary -v
    Discovering hosts using the mc method for 2 second(s) .... 0

    No request sent, we did not discover any nodes.Summary statistics for 0 nodes:

    The puppet application failed to run: undefined method `-' for nil:NilClass

    undefined method `-' for nil:NilClass (NoMethodError)
            from /usr/libexec/mcollective/mcollective/application/puppet.rb:184:in `sparkline_for_field'  <----
            from /usr/libexec/mcollective/mcollective/application/puppet.rb:281:in `summary_command'
            from /usr/libexec/mcollective/mcollective/application/puppet.rb:357:in `send'
            from /usr/libexec/mcollective/mcollective/application/puppet.rb:357:in `main'
            from /usr/lib/ruby/site_ruby/1.8/mcollective/application.rb:291:in `run'
            from /usr/lib/ruby/site_ruby/1.8/mcollective/applications.rb:23:in `run'
            from /usr/bin/mco:20
Merge pull request #21 from richardc/sparkline_exception

maint - check if there are results to sparkline first