Allow empty keyStore file for keyStoreTypes that do not require files

[kolbe]

This PR adds simple handling for an empty keyStore file, for use with KeyStoreManagers that do not require a defined keystore.

One such example is SPIFFE with java-spiffe: https://github.com/spiffe/java-spiffe.

[kolbe]

As long as the KeyManager implements a no-op KeyStoreSpi.engineLoad() (As java-spiffe does), a workaround for this issue is to create a "dummy" certificate (openssl req -x509 -new -out dummy.pem), load it into a "dummy" keystore (keytool -importcert -keystore dummy.keystore -storepass nopass -keypass nopass -file dummy.pem -noprompt), and use &clientCertificateKeyStoreUrl=file:dummy.keystore&clientCertificateKeyStorePassword=nopass in your JDBC connection URI.

Connector/J will try to load the keystore into the KeyManager, but the KeyManager simply ignores the attempt and will use key material it has acquired elsewhere.

[mysql-oca-bot]

Hi, thank you for submitting this pull request. In order to consider your code we need you to sign the Oracle Contribution Agreement (OCA). Please review the details and follow the instructions at http://www.oracle.com/technetwork/community/oca-486395.html
Please make sure to include your MySQL bug system user (email) in the returned form.
Thanks

[mysql-oca-bot]

Hi, thank you for your contribution. Please confirm this code is submitted under the terms of the OCA (Oracle's Contribution Agreement) you have previously signed by cutting and pasting the following text as a comment:
"I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it."
Thanks

[kolbe]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=98699 for updates.
Thanks