support blank prefix separator and use case instead
support blank prefix separator and use case instead

Used when tables are prefixed without a separtor e.g.
testTableOne
testTableTwo

can be backed up with:
./dynamodump.py -m 'backup' -r 'ap-southeast-2' --prefixSeparator '' -s 'test*'

and restored with:
./dynamodump.py -m ‘backup’ -r 'ap-southeast-2' --prefixSeparator '' -s 'test*' -d 'prod*'

Resulting restored tables would be:
prodTableOne
prodTableTwo

bchew/dynamodump,papiveron/dynamodump,bchew/dynamodump