fixed a potential NPE in SSLSocketAppenderBase

If the appender configuration did not specify an SSL configuration, an
NPE would occur in method start when creating the SSLContext.  This NPE
would be trapped and logged, but subsequently the getSocketFactory
method would be called and would return null, potentially causing an NPE
further up the stack.  The fix is to ensure that the appender always has
a default SSL configuration, even if one is not specified in the Joran
configuration.

Unit tests for the SSLConfiguration already validate that an SSLContext
can be successfully created from an SSLConfiguration that is merely
instantiated, but has none of its properties configured.