function fillEmptyCells() {
var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
var range = sheet.getRange("A1:A100");
var values = range.getValues();
for (var i = 0; i < values.length; i++) {
if (values[i][0] == "") {
values[i][0] = ".";
}
}
range.setValues(values);
}