Allow 1-to-0 transitions

### Description of the feature request:

Bazel supports 1-to-N transitions, but not if `N == 0`. Let me show an example:

```bzl
# //:BUILD.bazel
load(":defs.bzl", "apply_transition_and_fail")
load("@bazel_skylib//rules:common_settings.bzl", "string_flag")

string_flag(
name = "color",
build_setting_default = "default",
visibility = ["//visibility:public"],
)

apply_transition_and_fail(
name = "three",
colors = ["red", "green", "blue"],
)

apply_transition_and_fail(
name = "zero",
colors = [],
)

# //:defs.bzl
load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")

def _transition_impl(settings, attr):
return [
{
"//:color": color,
}
for color in attr.colors
]

_transition = transition(
implementation = _transition_impl,
inputs = [
],
outputs = [
"//:color",
],
)

def _apply_transition_and_fail_impl(ctx):
fail([
target[BuildSettingInfo].value
for target in ctx.split_attr._flag.values()
])

apply_transition_and_fail = rule(
implementation = _apply_transition_and_fail_impl,
attrs = {
"colors": attr.string_list(),
"_flag": attr.label(
default = "//:color",
cfg = _transition,
),
"_allowlist_function_transition": attr.label(
default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
),
},
)
```

If we run `bazel build //:three`, then the result will be as desired:
```
Error in fail: ["blue", "green", "red"]
```

But if we run `bazel build //:zero`, then the result will be surprising:
```
Error in fail: ["default"]
```

### Which category does this issue belong to?

Configurability

### What underlying problem are you trying to solve with this feature?

I want to make a 1-to-N transition, but N may occasionally be zero.

### What is the output of `bazel info release`?

release 6.1.0

### Any other information, logs, or outputs that you want to share?

Code for this proposal: https://github.com/alexander-shiryaev/bazel_issue/tree/main/feature_request/allow_1_to_0_transition

Start with the example in BUILD.bazel and defs.bzl, then review the linked proposal and reproduce bazel build //:three and bazel build //:zero. Done means a transition whose result list is empty produces no transitioned target rather than falling back to the default configuration.

Written by the indexing model from the issue text.