type/default: fix 'undefined index' in rrd_gen_graph

Fix php notices emitted when generating graphs of ping, sensors and
hddtemp plugins.
type: use empty to access ds_names

Fix some potential undefined index errors by using the empty function to
access ds_names in type other than default.
Merge pull request #21 from benoit-monin/fix_undefined_index

Fix undefined index for ds_names in type classes