Null-/undefined-safe tippy directive

[florianjung79]

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report 
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

Current behaviour

When using the the tippy directive, the assigned value has to be different from null or undefined. When using null or undefined an error is thrown. I know that it is possible to use isEnabled but would it be possible to introduce a global config that facilitates using the directive in a null-safe manner, i.e. when the assigned value is null or undefined no tooltip is shown.

[NetanelBasal]

You are welcome to create a PR