Yes, not many plugins implement that method. I think it is not common knowlegde with the plugin developers. I'm not sure if it is a bug or a feature ;-)
VST.NET assumes a parameter value range (externally) of [0, 1] if you use the VstParameterNormalizationInfo until a call is made to the GetParameterProperties method. I think this value range is correct because most plugin developers talk of this
parameter value range.
As a host I would assume the same value range of [0, 1] for all parameters of a plugin that returns null for the GetParameterProperties method. There are specific methods available for other meta info for parameters (GetParameterLabel, GetParameterName etc).
Hope it helps.