Support overwriting with a null `space_coinc_far` value using an empty string

Currently, if we get a new external event the ext_id and time_coinc_far info will be overwritten but not the space_coinc_far value. This could lead to some strange corner cases of bad behavior, so the safer strategy is use a different null value (None causes GraceDB to ignore the change) to fill this, such as an empty string.

changed milestone to %GWCelery 2.6.1 release

added External Triggers label

assigned to @brandon.piotrzkowski

This may be irrelevant now, since #921 (closed) addressed the root problem. This feature might still be helpful but need to think on it more.