Nullable field-types support

[workgena]

Official documentation does not describes this feature.

Some documentation can be found at ClickHouse/doc/example_datasets/nyc_taxi.md

In this PR I skipped Enum8 and Enum16, don't sure if they can be Nullable.

fixes #5

[workgena]

I'm not 100% sure about current implementation. It may broke something.
The other possible(safe) solution may be:

        def parse_value(type, value)
          if value
            case type
              when "UInt8", "UInt16", "UInt32", "UInt64", "Int8", "Int16", "Int32", "Int64"
                parse_int_value value
              when "Float32", "Float64"
                parse_float_value value
              when "String", "Enum8", "Enum16"
                parse_string_value value
              when /FixedString\(\d+\)/
                parse_fixed_string_value value
              when "Date"
                parse_date_value value
              when "DateTime"
                parse_date_time_value value
              when /Array\(/
                parse_array_value value
              else
                raise NotImplementedError, "Cannot parse value of type #{type.inspect}"
            end
          else
            parse_null_value(type)
          end
        end

        def parse_null_value(type)
          case type
            when "Nullable(UInt8)", "Nullable(UInt16)", "Nullable(UInt32)", "Nullable(UInt64)", "Nullable(Int8)", "Nullable(Int16)", "Nullable(Int32)",  "Nullable(Int64)"
              0
            when "Nullable(Float64)", "Nullable(Float32)"
              0.0
            when "Nullable(String)"
              ''
            when /Nullable\(FixedString\(\d+\)\)/
              ''
            else
              raise NotImplementedError, "Cannot parse value of type #{type.inspect}"
          end
        end