common: Allow null messages in Activator logging helpers

common: Allow null messages in Activator logging helpers

A very common message is e.getMessage(), which is not technically
guaranteed to be non-null, it would depend on the exception.

Allow the log messages to be null, we can convert them to empty
strings as needed.

Change-Id: I34695fd96368837547e8f74841668cb0b5c0c9a8
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/67183
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>