Drop Option wrapper on Manifest target vecs

[dwijnand]

As opposed to other manifest keys, such as authors, where a difference
may be drawn between:

authors = []

and the complete absence of the authors key the same cannot be said
for the target keys which are TOML Array of Tables types for which
the only way to define an empty array is by never defining any element
in it.

[rust-highfive]

r? @matklad

(rust_highfive has picked a reviewer for you, use r? to override)

[dwijnand]

this will clash with my #5335, but that's fine, I was looking to see if it was possible to simplify

let at_least_a_single_target_is_listed = toml_targets
	.map(|targets| !targets.is_empty())
    .unwrap_or(false);

to just

let at_least_a_single_target_is_listed = !toml_targets.is_empty();

feedback very welcome, I'm happy to iterate (and learn).

[matklad]

This is a horrible edge-case, but unfortunately one can do

test = []

[package]
name = "foo"
version = "0.1.0"
authors = ["an-developer@example.com"]

[dependencies]

[dwijnand]

Ah, I see what you mean. And now I understand better where the TOML docs explained that.

Shame.