Fix NRE on failed queue inflate

[HebaruSan]

Problem

If the queue inflator from #2798 encounters an error while inflating a netkan, an exception is thrown while trying to process the error:

544 [1] INFO CKAN.NetKAN.Processors.Inflator (null) - Using user-supplied cache at ckan_cache
1773837 [1] INFO CKAN.NetKAN.Processors.QueueHandler (null) - Inflating CustomDesignSpacesuits-Steampunk
1773840 [1] INFO CKAN.NetKAN.Validators.HasIdentifierValidator (null) - Validating that metadata contains an identifier
1773840 [1] INFO CKAN.NetKAN.Validators.KrefValidator (null) - Validating that metadata contains valid or null $kref
1773848 [1] INFO CKAN.NetKAN.Processors.Inflator (null) - Input successfully passed pre-validation
1774796 [1] INFO CKAN.NetKAN.Processors.QueueHandler (null) - Inflation failed, sending error: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
  at /build/mono-5.20.1.34/external/boringssl/ssl/handshake_client.c:1132
1774797 [1] FATAL CKAN.NetKAN.Program (null) - Object reference not set to an instance of an object

(That Object reference not set to an instance of an object line shouldn't be there. The rest are fine.)

Cause

If inflation fails, we don't have a ckan object, so we pass null to sendCkan:

CKAN/Netkan/Processors/QueueHandler.cs

Line 80 in d47a5b7

sendCkan(null, netkan, false, e.Message);

CKAN/Netkan/Processors/QueueHandler.cs

Line 101 in d47a5b7

private void sendCkan(Metadata ckan, Metadata netkan, bool success, string err = null)

This is passed to Program.CkanFileName:

CKAN/Netkan/Processors/QueueHandler.cs

Line 142 in d47a5b7

StringValue = Program.CkanFileName(ckan)

... which assumes it can't be null:

CKAN/Netkan/Program.cs

Lines 167 to 177 in d47a5b7

	internal static string CkanFileName(Metadata metadata)
	{
	return Path.Combine(
	Options.OutputDir,
	string.Format(
	"{0}-{1}.ckan",
	metadata.Identifier,
	metadata.Version.ToString().Replace(':', '-')
	)
	);
	}

The NRE would be thrown when we try to access .Identifier or .Version.

Changes

Now the FileName outgoing queue attribute is optional, only sent when the ckan parameter is non-null, because without it we don't know what the filename should be.

Fixes #2865.

[techman83]

@HebaruSan as always, love your work! I also have a sneaking suspicion that I might also not be account for a failure on the indexer side 😉