Clarify null/missing crs key (#225)

format-specs/geoparquet.md

@@ -65,7 +65,7 @@ The Coordinate Reference System (CRS) is an optional parameter for each geometry
 
 The CRS MUST be provided in [PROJJSON](https://proj.org/specifications/projjson.html) format, which is a JSON encoding of [WKT2:2019 / ISO-19162:2019](https://docs.opengeospatial.org/is/18-010r7/18-010r7.html), which itself implements the model of [OGC Topic 2: Referencing by coordinates abstract specification / ISO-19111:2019](http://docs.opengeospatial.org/as/18-005r4/18-005r4.html). Apart from the difference of encodings, the semantics are intended to match WKT2:2019, and a CRS in one encoding can generally be represented in the other.
 
-If CRS is not provided, all coordinates in the geometries MUST use longitude, latitude based on the WGS84 datum, and the default value is [OGC:CRS84](https://www.opengis.net/def/crs/OGC/1.3/CRS84) for CRS-aware implementations.
+If the `crs` key does not exist, all coordinates in the geometries MUST use longitude, latitude based on the WGS84 datum, and the default value is [OGC:CRS84](https://www.opengis.net/def/crs/OGC/1.3/CRS84) for CRS-aware implementations. Note that a missing `crs` key has different meaning than a `crs` key set to `null` (see below).
 
 [OGC:CRS84](https://www.opengis.net/def/crs/OGC/1.3/CRS84) is equivalent to the well-known [EPSG:4326](https://epsg.org/crs_4326/WGS-84.html) but changes the axis from latitude-longitude to longitude-latitude.