Support LDflex expressions that do not resolve to a thenable

[RubenVerborgh]

Since ldflex@2.0.0, expressions do not necessarily resolve to a thenable (in order to avoid infinite recursion). However, the React components still expect them to.

This leads to valid expressions such as ['https://ruben.verborgh.org/profile/#me'] erroring with:

src should be an LDflex path or string but is https://ruben.verborgh.org/profile/#me