[hf] safely handle absence of diagnostics phone numbers

contxt

• some sb instances might not have a diagnostics phone number, in which case we want logic that assumes a diagnostics phone number to not run

changes

• when there is no diagnostics phone number, that config is set to an empty string
• previously, some logic that did try to return early in the absence of a diagnostics phone number mistakenly checked for a null phone number to assess such absence. this MR fixes them to check for an empty string
• one function (#sendHealthchecks) did not perform such a check. This MR adds one