TextIO.withoutSharding sets shard template name to empty string instead of null

[kennknowles]

If TextIO.Write.withoutSharding() is called, it sets shard template name to empty string by calling withShardTemplateName(""). This causes an issue if custom filename policy is also set, since checks in expand() require that shardTemplateName is null if filenamePolicy is set.

Only workaround is currently to explicitly call withShardNAmeTemplate(null) after calling withoutSharding() resulting in an ugly code.

Imported from Jira BEAM-3082. Original Jira may contain additional context.
Reported by: sivu.