drivers: flash: shell: Fix printing partitions without labels

Commit df0f74d491c851b3e7e970f898c5c2fc3aab5b80 added a shell command to be
able to print out the partitions. However the label property is optional.

Print both the name and label if available.

Signed-off-by: Pieter De Gendt <pieter.degendt@basalte.be>