Allow installing modules without download_size

[HebaruSan]

Problem

Some old modules don't have download_size set:

https://github.com/KSP-CKAN/CKAN-meta/blob/master/Toolbar/Toolbar-1.7.9.ckan

But it's just a simple int, so that causes it to be set to 0:

CKAN/Core/Types/CkanModule.cs

Lines 256 to 257 in ab714c2

	[JsonProperty("download_size")]
	public long download_size;

When that happens, we can't install the module because it thinks the size "should be 0":

CKAN/Core/Net/NetModuleCache.cs

Lines 117 to 120 in ab714c2

	// Check file size
	if (fi.Length != module.download_size)
	throw new InvalidModuleFileKraken(module, path,
	$"{module}: {path} has length {fi.Length}, should be {module.download_size}");

https://ci.ksp-ckan.org/job/CKAN-meta/939/console

CKAN.InvalidModuleFileKraken: Toolbar 1.7.9: /tmp/tmp44f9291a.tmp has length 63906, should be 0
  at CKAN.NetModuleCache.Store (CKAN.CkanModule module, System.String path, System.String description, System.Boolean move) [0x00061] in <cf516cc8950f43f6b8db7627f6e8d84f>:0 
  at CKAN.NetAsyncModulesDownloader.ModuleDownloadsComplete (CKAN.NetModuleCache cache, System.Uri[] urls, System.String[] filenames, System.Exception[] errors) [0x00031] in <cf516cc8950f43f6b8db7627f6e8d84f>:0 

This has blocked validation of KSP-CKAN/CKAN-meta#1356

Changes

Now we only enforce the size requirement if download_size is positive.