Configatron::Proc#call returns @val only when val is nil

[t-miyamoto-jp]

#118

Configatron::Dynamic#call returns nil when value is false.

proc = Configatron::Proc.new { false }
proc.call
=> false

dynamic = Configatron::Dynamic.new { false }
dynamic.call
=> nil

Dynamic#call and Proc#call should return the same value, so I changed the condition.