fix: no early return in loadTransportStreamBuilder

[viceice]

Don't return early in loadTransportStreamBuilder, it'll return undefined instead if the expected function.
I'll also fixed the type annotation to be a promise. 😉

• closes Error: "fn is not a function" while setting up a pino transports since v9.2.1 #2012
• regression of forward esm syntax errors #1990

[mcollina]

lgtm

[viceice]

🤔 the failed test is unrelated to my changes 🤔