Cannot play SuperDirt samples 808 and 909 with an index

[pierredewilde]

Pa * d('808') is played correctly but Pa * d('808:0') generated an error:

ValueError: Cannot pattern an empty sequence: []

Same for Pa * d('909') and Pa * d('909:0').

All other samples play correctly.

Please note that 808 and 909 are the only SuperDirt samples without at least a letter in their names.