fix(agent): disallow update if prompt is empty

[fanlv]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ent/singleagent/internal/dal/single_agent_draft.go	0.00%	9 Missing ⚠️
...ain/agent/singleagent/service/single_agent_impl.go	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
.../domain/agent/singleagent/repository/repository.go	0.00% <ø> (ø)
...ain/agent/singleagent/service/single_agent_impl.go	0.00% <0.00%> (ø)
...ent/singleagent/internal/dal/single_agent_draft.go	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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