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

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](https://issues.apache.org/jira/browse/BEAM-3082). Original Jira may contain additional context.
Reported by: sivu.

Start with TextIO.Write.withoutSharding() and follow how it calls withShardTemplateName(), then inspect expand() and the filenamePolicy checks. Done means withoutSharding leaves shardTemplateName as null so a custom filename policy works without an extra call to withShardNameTemplate(null).

Written by the indexing model from the issue text.