Add catch for empty log_evidence_err array

I ran into an issue when trying to use the ResultList object where all the log evidence errors in my individual runs were infinite so I wasn't able to use the combine() method. I've just added a catch that if this array is empty, return nan as the new result.log_evidence_err. I could also see the value in returning the standard deviation of the evidences from the individual runs. What do the rest of you think?

bilby/core/result.py

@@ -1320,7 +1320,10 @@ class ResultList(list):
         else:
             result.log_bayes_factor = result.log_evidence - result.log_noise_evidence
         log_errs = [res.log_evidence_err for res in self if np.isfinite(res.log_evidence_err)]
-        result.log_evidence_err = logsumexp(2 * np.array(log_errs), b=1. / len(self))
+        if len(log_errs) > 0:
+            result.log_evidence_err = logsumexp(2 * np.array(log_errs), b=1. / len(self))
+        else:
+            result.log_evidence_err = np.nan
         result_weights = np.exp(log_evidences - np.max(log_evidences))
         posteriors = []
         for res, frac in zip(self, result_weights):