Allow parent span to be null

[dyladan]

Is your feature request related to a problem? Please describe.
The following pattern is used in almost every plugin:

const span = tracer.startSpan(spanName, {
    parent: tracer.getCurrentSpan() || undefined,
    // ...
});

This is because getCurrentSpan currently returns Span | null but parent is an optional Span (parent?: Span).

Describe the solution you'd like

Propose changing the signature of parent to parent?: Span | null.

Describe alternatives you've considered
Continuing to use the parent: tracer.getCurrentSpan() || undefined, pattern

Additional context
#525 (comment)

[dyladan]

pls assign me

[mayurkale22]

Closing via #569