[NEW FEAT] Skip merge process for lines with empty recipient

[ttsukagoshi]

Is your feature request related to a problem? Please describe.

There are cases when the user would like to use a spreadsheet with empty lines, with just the line number entered. When the Gmail draft is created for an empty line, Apps Script returns an error message:

Exception: 無効なメール: NA
     at mailMerge (group-merge:1135:28)
     at createDraftEmails (group-merge:877:5)
     at __GS_INTERNAL_top_function_call__.gs:1:8

Describe the solution you'd like

Add process to skip lines with an empty recipient cell.

Describe alternatives you've considered

The Spreadsheet.getDataRange() gets the range of cells with data, equivalent to the Ctrl + A shortcut key, so this should not be a problem unless the user has extra rows with limited field(s) filled in, e.g. row number, or if there are empty rows in the middle of the table.
The current workaround would be to edit the spreadsheet so that no empty lines exist in the spreadsheet.

Additional context

NA