Fix: Snowflake NPE on bad arguments

[misolt]

Running, for example:

my-dumper-executable --connector snowflake --assessment

used to cause a NPE (due to the missing --user flag):

********************************************************************
java.lang.NullPointerException
        at java.util.Hashtable.put(Hashtable.java:460)
        at com.google.edwmigration.dumper.application.dumper.connector.snowflake.AbstractSnowflakeConnector.createUserPasswordDataSource(AbstractSnowflakeConnector.java:130)
        at com.google.edwmigration.dumper.application.dumper.connector.snowflake.AbstractSnowflakeConnector.open(AbstractSnowflakeConnector.java:95)
        at com.google.edwmigration.dumper.application.dumper.MetadataDumper.run(MetadataDumper.java:143)
        at com.google.edwmigration.dumper.application.dumper.MetadataDumper.run(MetadataDumper.java:89)
        at com.google.edwmigration.dumper.application.dumper.Main.main(Main.java:62)
********************************************************************

Make it throw a usage exception instead:

ERROR com.google.edwmigration.dumper.application.dumper.Main - Required username was not provided.
Please use the '--user' flag to provide the username.

Additionally, this improves test coverage (don't listen to the bot).

[github-actions]

Code Coverage Report

Overall Project	61.61% -0.24%	🍏
Files changed	59.42%	🍏

File	Coverage
AbstractSnowflakeConnector.java	73.18% -21.88%	🍏