fix: guard empty context array in spec command under set -u

[satyaborg]

Summary

• devloop spec with no source material crashed with context_items[@]: unbound variable on macOS bash 3.2, because set -uo pipefail rejects expanding an empty array via "${context_items[@]}".
• Switched the call site to the bash 3.2-safe guard ${context_items[@]+"${context_items[@]}"}, so an empty array expands to nothing and resolve_spec_context correctly takes its cold-start (zero-args) path.

Test plan

• bash scripts/devloop_test.sh — all pass (incl. 100% function coverage)
• bash -n devloop syntax check
• Reproduced the empty-array-under-set -u failure and confirmed the new pattern clears it