ICSLT: Use NA table if data files are not available

inst/extdata/MortalityTables_ICSLT.R

@@ -44,11 +44,13 @@ if (!is.null(getOption("MortalityTables.ICSLT2018"))) {
   ICSLT2018 = readICSLT(getOption("MortalityTables.ICSLT2018"), name = "ICSLT 2018", year = 2018, coloffset = 0)
 } else {
   message("Path to the ICSLT©2018.xlsx data file (option 'MortalityTables.ICSLT2018' not set, ICSLT2018 will not be available!")
+  ICSLT2018 = mortalityTable.NA
 }
 if (!is.null(getOption("MortalityTables.ICSLT2013"))) {
   ICSLT2013 = readICSLT(getOption("MortalityTables.ICSLT2013"), name = "ICSLT 2013", year = 2013, coloffset = 1)
 } else {
   message("Path to the ICSLT©2013.xlsx data file (option 'MortalityTables.ICSLT2013' not set, ICSLT2013 will not be available!")
+  ICSLT2013 = mortalityTable.NA
 }
 # if (!is.null(getOption("MortalityTables.ICSLT2008"))) {
 #   ICSLT2008 = readICSLT(getOption("MortalityTables.ICSLT2008"), name = "ICSLT 2008", year = 2008)