[LOG4J2-3548] Don't replace null password with default if provided

[kristofarkas]

Password-less key stores require passing null as password. A recent change prevented that, replacing null password with a default value. This PR reverts that change, allowing null password to be used again.

https://issues.apache.org/jira/browse/LOG4J2-3548

[ppkarwasz]

Hi @kristofarkas,

I would appreciate if you could add some tests to prevent a regression in the future. I imagine we would like to check these three cases:

• a PKCS12 keystore with a non-empty password,
• a PKCS12 keystore with an empty password,
• a keystore, which requires both password and location to be null (e.g. Windows-MY and Windows-ROOT).

While your PR fixes the usage of Windows-MY/ROOT inside a Log4j2 configuration file, there is another case that you could consider fixing: the SslConfigurationFactory contains an SslConfiguration used to download a Log4j2 configuration file itself over HTTPS. Unless I am mistaken, this configuration will not work with Windows-MY, since it requires the property "log4j2.keyStoreLocation" to be non-null.

[ppkarwasz]

@kristofarkas, thank you for your contribution.