Handle enums with zero or one variants when deriving `BoundedArbitrary`.

Resolves #4170

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Auto generated release notes:

## What's Changed
* Edit quantifiers' documentation. by @thanhnguyen-aws in
#4142
* Fix the bug of using multiple hidden variables for the prev of the
same Expr by @thanhnguyen-aws in
#4150
* Remove `assess` subcommand by @carolynzech in
#4111
* Optimize goto binary exporting in `cprover_bindings` by
@AlexanderPortland in #4148
* Add the option to generate performance flamegraphs by
@AlexanderPortland in #4138
* Fix the bug: Static union values can panic Kani by @thanhnguyen-aws in
#4112
* Update toolchain to 2025-06-13 by @carolynzech in
#4152
* Automatic cargo update to 2025-06-16 by @github-actions in
#4156
* Major-version update cargo dependencies by @tautschnig in
#4158
* Upgrade Rust toolchain to 2025-06-16 by @tautschnig in
#4157
* Bump tests/perf/s2n-quic from `3129ad5` to `c6e694e` by @dependabot in
#4160
* Bump tests/perf/s2n-quic from `c6e694e` to `b1b5bf8` by @dependabot in
#4164
* Upgrade Rust toolchain to 2025-06-17 by @tautschnig in
#4163
* Automatic cargo update to 2025-06-23 by @github-actions in
#4172
* Stub panics during MIR transformation by @AlexanderPortland in
#4169
* Bump tests/perf/s2n-quic from `b1b5bf8` to `32ba87d` by @dependabot in
#4175
* Handle enums with zero or one variants by @zhassan-aws in
#4171
* Introduce compiler timing script & CI job by @AlexanderPortland in
#4154
* Upgrade Rust toolchain to 2025-06-18 by @tautschnig in
#4166
* Cache dependencies for CI jobs by @AlexanderPortland in
#4181
* Autoharness: Derive `Arbitrary` for structs and enums by @carolynzech
in #4167
* Upgrade Rust toolchain to 2025-06-27 by @tautschnig in
#4182
* Include wget in dependencies by @zhassan-aws in
#4183
* Automatic cargo update to 2025-06-30 by @github-actions in
#4186
* Add support for loop assigns in loop contracts by @thanhnguyen-aws in
#4174
* Upgrade toolchain to 06/30 by @carolynzech in
#4188
* Optimize reachability with non-mutating global passes by
@AlexanderPortland in #4177
* Bump tests/perf/s2n-quic from `32ba87d` to `b8f8cca` by @dependabot in
#4190
* Bump ncipollo/release-action from 1.16.0 to 1.18.0 by @dependabot in
#4191
* Upgrade toolchain to 07/02 by @carolynzech in
#4195
* Automatic Derivation Fixes by @carolynzech in
#4194


**Full Changelog**:
kani-0.63.0...kani-0.64.0

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Handle enums with zero or one variants when deriving `BoundedArbitrary`.

Resolves #4170

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.