fix null attr access

[AhmedMozaly]

I'm using blessed as part of PM2 setup Noticed that PM2 monit command works for some apps but not for other, after checking the issue. It was this error

TypeError: Cannot read properties of null (reading 'slice')
    at /Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/program.js:2543:35
    at Array.forEach (<anonymous>)
    at Program._attr (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/program.js:2542:11)
    at Element._parseTags (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/widgets/element.js:498:26)
    at Element.parseContent (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/widgets/element.js:393:22)
    at Box.<anonymous> (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/widgets/element.js:184:10)
    at EventEmitter._emit (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/events.js:94:20)
    at EventEmitter.emit (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/events.js:117:12)
    at emit (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/widgets/node.js:109:15)
    at Node.insert (/Users/ahmed/.nvm/versions/node/v18.16.0/lib/node_modules/pm2/node_modules/blessed/lib/widgets/node.js:111:5)

This PR should fix this noisy issue

[MrFiregore]

happens when there are square brackets as tags, but only opening brackets

for example when passing this text:

> [email protected] build C:\www\open-con-jetbrains
> pnpm run build:clean && pnpm run build:tsc && pnpm run build:di


> [email protected] build:clean C:\www\open-con-jetbrains
> del-cli --force --verbose dist

C:www\open-con-jetbrains\dist

> [email protected] build:tsc C:\www\open-con-jetbrains
> tsc -p tsconfig.json


> [email protected] build:di C:\www\open-con-jetbrains
> copyfiles -V -u 1 ./src/**/*.{json,yaml,html,png,hbs} ./dist

unglobed path: ./src/config.default.json

Note that {json,yaml,html,png,hbs} is not a tag.