Fix unit tests aborting on bash 3.2

[corioliskraft]

How to categorize this PR?

/kind bug

What this PR does / why we need it:

Script hack/test.sh sets nounset at line 4 and expands test_flags at line 27. Outside
CI that array is empty, and bash 3.2 treats the expansion of an empty array as an unbound
variable. Therefore make verify aborts before it runs a test:

./hack/test.sh ./cmd/... ./pkg/...
> Unit Tests
./hack/test.sh: line 27: test_flags[@]: unbound variable
make: *** [test] Error 1

macOS ships bash 3.2.57 as /bin/bash, so every contributor on a stock macOS machine hits
this at CONTRIBUTING step 4.

This PR guards the expansion, so it disappears when the array is empty. This matches the
${timeout_flag:+...} guard in the same line. Prow is not affected either way, because
there CI and ARTIFACTS are set and test_flags is never empty.

Which issue(s) this PR fixes:

Fixes #1504

Special notes for your reviewer:

The behavior of the guard on bash 3.2.57:

$ /bin/bash -c 'set -u; a=(); echo "before"; echo ${a[@]+"${a[@]}"}; echo "after"'
before

after
$ /bin/bash -c 'set -u; a=(one two); printf "[%s]\n" ${a[@]+"${a[@]}"}'
[one]
[two]

The second command shows that the elements stay separate arguments. The form uses + and
not :+, because the test must be "set" and not "set and not empty".

make verify passes on macOS 15.7.7, arm64, with bash 3.2.57.

Breaking changes:

None.

[ske-prow]

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[ske-prow]

Welcome @corioliskraft!

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[ske-prow]

Hi @corioliskraft. Thanks for your PR.

I'm waiting for a stackitcloud member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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