(GH-164) Fix "The path is not of a legal form"

When file_system.get_full_path is passed an empty string resulting from grabbing
the directory name, it would throw the error above. Instead, allow it to return
immediately, then use the current directory instead.

This usually happens when calling `choco pack some.nuspec`.

src/chocolatey/infrastructure.app/services/NugetService.cs

@@ -134,6 +134,7 @@ public void pack_run(ChocolateyConfiguration config)
         {
             string nuspecFilePath = validate_and_return_package_file(config, Constants.ManifestExtension);
             var nuspecDirectory = _fileSystem.get_full_path(_fileSystem.get_directory_name(nuspecFilePath));
+            if (string.IsNullOrWhiteSpace(nuspecDirectory)) nuspecDirectory = _fileSystem.get_current_directory();
 
             IDictionary<string, string> properties = new Dictionary<string, string>();
             // Set the version property if the flag is set

src/chocolatey/infrastructure/filesystem/DotNetFileSystem.cs

@@ -66,6 +66,8 @@ public string combine_paths(string leftItem, params string[] rightItems)
 
         public string get_full_path(string path)
         {
+            if (string.IsNullOrWhiteSpace(path)) return path;
+
             return Path.GetFullPath(path);
         }