Avoid NPE for unspecified keystore key

[findinpath]

Fixes #976

[findinpath]

After testing this change with Trino, the error occuring is way more misleading.

Caused by: RuntimeException: UnrecoverableKeyException: Get Key failed: Given final block not properly padded. Such issues can arise if a bad key is used during decryption.
	at HttpServerProvider.get(HttpServerProvider.java:173)
	at HttpServerProvider.get(HttpServerProvider.java:46)
	at ProviderInternalFactory.provision(ProviderInternalFactory.java:86)
	at BoundProviderFactory.provision(BoundProviderFactory.java:72)
	at ProviderInternalFactory$1.call(ProviderInternalFactory.java:67)

Loading a password protected keystore with a NULL password does not throw an exception:

        String keystorePassword = null;
        try (InputStream in = new FileInputStream(new File("presto-master.jks"))) {
            KeyStore keystore = KeyStore.getInstance("JKS");
            keystore.load(in, Optional.ofNullable(keystorePassword).map(String::toCharArray).orElse(null));
        }

Detailed reasoning behind this choice:

https://github.com/AdoptOpenJDK/openjdk-jdk11/blob/master/src/java.base/share/classes/sun/security/pkcs12/PKCS12KeyStore.java#L2044-L2051

https://github.com/openjdk/jdk17/blob/master/src/java.base/share/classes/sun/security/pkcs12/PKCS12KeyStore.java#L2065-L2087

NOTE that in case of providing an invalid password we'll receive here

Exception in thread "main" java.io.IOException: keystore password was incorrect
	at java.base/sun.security.pkcs12.PKCS12KeyStore.engineLoad(PKCS12KeyStore.java:2159)

A possibility to go around this is to have a special handling in case of NULL keystore password by going over the aliases and check whether the certificates can be retrieved

            Enumeration<String> enumeration = keystore.aliases();
            while(enumeration.hasMoreElements()) {
                String alias = enumeration.nextElement();
                Certificate certificate = keystore.getCertificate(alias);
                 if (certificate == null){
                       // we're dealing with an invalid password
                 }
            }

[electrum]

Your suggested code sounds better. In the case where we need a password but one isn't provided, it seems better to fail up front with a good error message.

[findinpath]

I'll refresh the PR and come up with a better version of this PR.