Avoid null ksp_version in Netkan

[HebaruSan]

Problem

#2553 tried to fix a problem with setting "ksp_version": "any" in a netkan that also has a $vref. The change worked insofar as netkan was able to generate a .ckan file instead of erroring out, but the script that checks the .ckan file against the schema found a problem: ksp_version was null, and that's not allowed.

    "ksp_version": null,

Validating built/CommunityTerrainTexturePack-1-1.0.4.ckan.. Failed! See below for error description.
None is not of type u'string'

Failed validating u'type' in schema[u'properties'][u'ksp_version']:
    {u'description': u'A version of KSP',
     u'pattern': u'^(any|[0-9]+\\.[0-9]+(\\.[0-9]+)?)$',
     u'type': u'string'}

On instance[u'ksp_version']:
    None

This is blocking KSP-CKAN/NetKAN#6791.

Cause

KspVersion.Any.ToString() returns null because of this:

CKAN/Core/Versioning/KspVersion.cs

Line 774 in 5b54850

return s.Equals(string.Empty) ? null : s;

(This is kind of gross, since the string "any" is specifically allowed in the spec and could be used here, but I'm reluctant to mess with such a core assumption.)

Then when we parse "any" and return KspVersion.Any, these lines then set ksp_version to null:

CKAN/Netkan/Transformers/AvcTransformer.cs

Lines 145 to 152 in 5b54850

	if (kspMin != null && kspMax != null)
	{
	// If we have both a minimum and maximum KSP version...
	if (kspMin.CompareTo(kspMax) == 0)
	{
	// ...and they are equal, then just set ksp_version
	json["ksp_version"] = kspMin.ToString();
	}

Changes

The if checks in these lines are in charge of keeping null out of the lists of possible min and max versions:

CKAN/Netkan/Transformers/AvcTransformer.cs

Lines 122 to 132 in 5b54850

	if (existingKspMin != null)
	kspMins.Add(existingKspMin);
	if (avcKspMin != null)
	kspMins.Add(avcKspMin);
	if (existingKspMax != null)
	kspMaxes.Add(existingKspMax);
	if (avcKspMax != null)
	kspMaxes.Add(avcKspMax);

Now they're updated to also prevent KspVersion.Any from being added to those lists. This is done with a new static helper function KspVersion.IsNullOrAny in the style of string.IsNullOrEmpty. This will cause the AVC transformer to not make any changes, and the "any" value from the original netkan will flow through to the .ckan file, which is valid according to the spec.