Add default value to input mapping when storage.input.tables key missing

[ujovlado]

Fixes #13

For sync actions an error output is silenced. And due to missing key storage, this error won't be shown:

[2018-06-19 20:14:31] wr-db-redshift.ERROR: Undefined index: storage 
{"errFile":"/code/src/Keboola/DbWriter/Redshift/Application.php","errLine":35,"trace":
[{"file":"/code/src/Keboola/DbWriter/Redshift/Application.php","line":35,"function":"{closure}","args":
[8,"Undefined index: storage","/code/src/Keboola/DbWriter/Redshift/Application.php",35,{"config":
{"action":"testConnection","parameters":{"db":
{"host":"192.168.1.227","port":"5432","database":"test","user":"test","password":"test","schema":"test"},
"data_dir":"./data","writer_class":"Redshift"}},"logger":"[object] (Keboola\\DbWriter\\Logger: 
{})","configDefinition":"[object] (Keboola\\DbWriter\\Redshift\\Configuration\\ConfigDefinition: 
{})","validate":"[object] (Closure: {})","parameters":{"db":
{"host":"192.168.1.227","port":"5432","database":"test","user":"test","password":"test","schema":"test"},
"data_dir":"./data","writer_class":"Redshift","tables":[]}}]},
{"file":"/code/run.php","line":33,"function":"__construct","class":"Keboola\\DbWriter\\Redshift\\Application"
,"type":"->","args":[{"action":"testConnection","parameters":{"db":
{"host":"192.168.1.227","port":"5432","database":"test","user":"test","password":"test","schema":"test"},
"data_dir":"./data","writer_class":"Redshift"}},"[object] (Keboola\\DbWriter\\Logger: {})","[object] 
(Keboola\\DbWriter\\Redshift\\Configuration\\ConfigDefinition: {})"]}]} []

This PR will set default value for input mapping, so Undefined index error will be fixed and sync actions will be fixed.

[ujovlado]

@MiroCillik ked pustis writer bez tabuliek, tak to spadne rovnako. Takze asi by to to tam nechal v oboch pripadoch.

Skonci to ako:

{"status":"success","uploaded":[]}

ked tak sa to moze doriesit v dalsej issue - nech to hodi napr. user error, ked nemas tabulky