Meaning of "I trust the journey was uneventful"

The phrase is stating that the speaker hopes that the person had a journey that involved no problems. The hope is that no negative events occurred, rendering the journey uneventful. In this sense the meaning of uneventful is maintained, albeit only so far as it applies to negative occurrences.

"I trust [that] the journey was uneventful"

OED:

Trust (v.) 5.a. transitive. To have confidence that something desired is, or will be, the case. With clause (esp. that-clause) as object […] to hope, expect.

1937 R. Stout Red Box ix. 136 ‘I trust that we are still brothers-in-arms?—’ ‘Absolutely. Pals.’

2000 R. W. Holder Taunton Cider & Langdons xvii. 90 You will, I trust, forgive me if we stick for a moment to the bare essentials.

Macmillan Dictionary:

I trust (that) – phrase, spoken, formal

DEFINITION 1.1 **used for saying that you hope and expect that something is true. ** “I trust that you’ve already completed the paperwork, Mrs Williams.”

The background assumption here is that this is the kind of journey that is undertaken solely for the purpose of getting to the destination, without expecting any interesting, exciting experiences along the way. Think of, for example, a train journey that one spends reading a book, because there is nothing interesting to see through the windows. When such trip proceeds as expected, it is uneventful; what would make it eventful would be something like breakdowns, delays, accidents, etc. If one is met by somebody at the end of such a trip, it is quite natural for the person to say 'I trust the journey was uneventful', because in that context, it amount to saying that the person trusts that there were no breakdowns, delays, accidents, etc. on the journey.

'I trust the journey was uneventful' is not something that would be said when meeting someone who has completed a journey that was supposed to be interesting and exciting, such as a train ride that was undertaken specifically for the purpose of admiring the scenery along the route.