[FEATURE] Avoid zero value for direction type in custom schema

[zstadler]

Is your feature request related to a problem? Please describe.
The custom YAML schema defines the direction type as

• direction - Maps "-1" to -1, "1" "yes" or "true" to 1, and everything else to 0. See Key:oneway.

As a result, the default value 0 is set for the vast majority of highway features resulting in an unnecessary increase in tile size.

Describe the solution you'd like
Limit the possible values for the direction to 1 and -1 leaving the attribute undefined otherwise:

• direction - Maps "-1" to -1, "1" "yes" or "true" to 1, and everything else leaves the attribute unset (null) . See Key:oneway.

Describe alternatives you've considered
Calculate this attribute using an expression of type int

    - key: oneway
      # type: direction # would add 0 elsewhere
      type: int
      value:
        1:
          oneway: ['yes', '1', 'true']
        -1:
          oneway: '-1'

Additional context
OMT and planetiler-openmaptiles also avoid adding oneway: 0 attributes

[msbarry]

@phanecak-maptiler do you know if direction=0 is strictly necessary for openmaptiles schema? I'd be up for changing the default behavior of direction conversion from -1/0/1 to -1/null/1 but that would affect openmaptiles as well.

[zstadler]

OMT emits only oneway <> 0:

CASE WHEN is_oneway <> 0 THEN is_oneway::int END AS oneway,

[zstadler]

Another option to address this issue in a more general way is to add a nullif: option to attributes. If the calculated/final value of the attribute is equal to any of the nullif values, the attribute would be omitted from the tile feature. For example:

- key: oneway
  type: direction
  nullif: 0                    # Avoid oneway=0
- key: bridge
  nullif: ['no', 'No', 0]      # Avoid several flavors of non-bridges
- key: is_bridge
  tag_value: bridge
  type: boolean
  nullif: false                # Avoid is_bridge=false