fix some type instabilities introduced by #39607

[aviatesk]

After #39607, Iterators.peel can return nothing and the return type
can be more inaccurate than before for certain iterative inputs where
inference doesn't understand its length, e.g.
Iterators.peel(::<:AbstractString).

The following JET analysis shows an example of this type instability:

═════ 2 possible errors found ═════
┌ @ coreio.jl:4 Base.println(Core.tuple(Core.typeassert(Base.stdout, Base.IO)), xs...)
│┌ @ strings/io.jl:73 Base.print(Core.tuple(io), xs, Core.tuple("\n")...)
││┌ @ strings/io.jl:46 Base.print(io, x)
│││┌ @ show.jl:1283 Base.show_unquoted(Base.IOContext(io, Base.=>(:unquote_fallback, false)), ex, 0, -1)
││││┌ @ show.jl:1634 Base.show_unquoted_quote_expr(io, Base.getproperty(ex, :value), indent, prec, 0)
│││││┌ @ show.jl:1651 Base.isidentifier(sym)
││││││┌ @ show.jl:1339 Base.isidentifier(Base.string(s))
│││││││┌ @ show.jl:1335 Base.indexed_iterate(Base.Iterators.peel(s), 1)
││││││││┌ @ tuple.jl:92 Base.iterate(I)
│││││││││ no matching method found for call signature (Tuple{typeof(iterate), Nothing}): Base.iterate(I::Nothing)
││││││││└───────────────
│││││││┌ @ show.jl:1335 Base.indexed_iterate(Base.Iterators.peel(s), 2, _3)
││││││││┌ @ tuple.jl:97 Base.iterate(I, state)
│││││││││ no matching method found for call signature (Tuple{typeof(iterate), Nothing, Int64}): Base.iterate(I::Nothing, state::Int64)
││││││││└───────────────
Core.Const(nothing)

This PR adds some manual annotations and seems to fix some regressions
within Base.

[aviatesk]

When there is no objection against this, I'm going to merge this tmrw.