(maint) Improve output when exporting releases

When exporting releases, if not providing a specific tagName, the output
was:

Exporting release null

Which isn't very helpful.

This commit changes this output so that if the tagName is included,
include it in the output, if it isn't, assume that all releases are
being exported, and output this instead.

Author: gep13

src/GitReleaseManager.Core/Commands/ExportCommand.cs

@@ -18,7 +18,15 @@ public ExportCommand(IVcsService vcsService, ILogger logger)
 
         public async Task<int> ExecuteAsync(ExportSubOptions options)
         {
-            _logger.Information("Exporting release {TagName}", options.TagName);
+            if (string.IsNullOrEmpty(options.TagName))
+            {
+                _logger.Information("Exporting all releases");
+            }
+            else
+            {
+                _logger.Information("Exporting release {TagName}", options.TagName);
+            }
+
             var releasesContent = await _vcsService.ExportReleasesAsync(options.RepositoryOwner, options.RepositoryName, options.TagName, options.SkipPrereleases).ConfigureAwait(false);
 
             using (var sw = new StreamWriter(File.Open(options.FileOutputPath, FileMode.OpenOrCreate)))