[QGIS] Handle null Z values internally as NaN

[haubourg]

Following default z changes to represent NULL value using a configurable integer, the idea suggested by @m-kuhn is to handle NaN in QGIS API and map it to a default value when writing to datasource via providers. Currently most providers do not handle NULL or NaN values for Z, and mapping Z to a default integer (ex: 9999) is the only way to go.