Multi-sheet conditional formatting crashes when not all sheets have rules
Two sheets, conditional formatting rules only on the first sheet. Passing an empty array [] for the second sheet works, but undefined or omitting it crashes.
```
node -e '
var w = require("write-excel-file/node")
var cf = new Array(2)
cf[0] = [{
cellRange: {from:{row:1,column:1}, to:{row:1,column:1}},
condition: {operator:"=", value:"a"},
style: {backgroundColor:"#ff0000"}
}]
w([[[{value:"a",type:String}]], [[{value:"b",type:String}]]],
{sheets:["S1","S2"], conditionalFormatting:cf, filePath:"/tmp/b.xlsx"}
).catch(function(e) { console.log(e.message) }) '
```
`TypeError: Cannot read properties of undefined (reading "length")'`
issue