embulkで 'Setting null to a task field is not allowed. Use Optional<T> to represent null.' が出たときにやったこと

embulk-input-mysqlを使っていて、以下のようなエラーが出た。

org.embulk.exec.PartialExecutionException: org.embulk.config.ConfigException: com.fasterxml.jackson.databind.JsonMappingException: Setting null to a task field is not allowed. Use Optional<T> to represent null.
	at org.embulk.exec.BulkLoader$LoaderState.buildPartialExecuteException(BulkLoader.java:340)
	at org.embulk.exec.BulkLoader.doRun(BulkLoader.java:566)
	at org.embulk.exec.BulkLoader.access$000(BulkLoader.java:35)
	at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:353)
	at org.embulk.exec.BulkLoader$1.run(BulkLoader.java:350)
	at org.embulk.spi.Exec.doWith(Exec.java:22)
	at org.embulk.exec.BulkLoader.run(BulkLoader.java:350)
	at org.embulk.EmbulkEmbed.run(EmbulkEmbed.java:178)
	at org.embulk.EmbulkRunner.runInternal(EmbulkRunner.java:292)
	at org.embulk.EmbulkRunner.run(EmbulkRunner.java:156)
	at org.embulk.cli.EmbulkRun.runSubcommand(EmbulkRun.java:433)
	at org.embulk.cli.EmbulkRun.run(EmbulkRun.java:90)
	at org.embulk.cli.Main.main(Main.java:64)
	Suppressed: java.lang.NullPointerException
		at org.embulk.exec.BulkLoader.doCleanup(BulkLoader.java:463)
		at org.embulk.exec.BulkLoader$3.run(BulkLoader.java:397)
		at org.embulk.exec.BulkLoader$3.run(BulkLoader.java:394)
		at org.embulk.spi.Exec.doWith(Exec.java:22)
		at org.embulk.exec.BulkLoader.cleanup(BulkLoader.java:394)
		at org.embulk.EmbulkEmbed.run(EmbulkEmbed.java:181)
		... 5 more
Caused by: org.embulk.config.ConfigException: com.fasterxml.jackson.databind.JsonMappingException: Setting null to a task field is not allowed. Use Optional<T> to represent null.
	at org.embulk.config.ModelManager.readObjectWithConfigSerDe(ModelManager.java:78)
	at org.embulk.config.DataSourceImpl.loadConfig(DataSourceImpl.java:197)
	at org.embulk.input.jdbc.AbstractJdbcInputPlugin.transaction(AbstractJdbcInputPlugin.java:191)
	at org.embulk.exec.BulkLoader.doRun(BulkLoader.java:507)
	... 11 more
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Setting null to a task field is not allowed. Use Optional<T> to represent null.
	at org.embulk.config.TaskSerDe$TaskDeserializer.deserialize(TaskSerDe.java:154)
	at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:3708)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2005)
	at org.embulk.config.ModelManager.readObjectWithConfigSerDe(ModelManager.java:73)
	... 14 more

Error: org.embulk.config.ConfigException: com.fasterxml.jackson.databind.JsonMappingException: Setting null to a task field is not allowed. Use Optional<T> to represent null.

ローカルのMySQLのパスワードを空にしていたのだが、環境変数として渡す際に以下のようにダブルクォート（or シングルクォート）で囲む必要があったらしい。（Dockerで実行していて--env-fileで.envを渡していた）

.env

MYSQL_PASSWORD=""

これでエラーが出ずに動くようになった。