BUG: SeedGroup fails on seed prompts with metadata=None #3508

Describe the bug

SeedPrompt.metadata is typed dict[str, Any] | None, so it can be None. SeedGroup passes it straight into MessagePiece(prompt_metadata=...), which only accepts a dict. So next_message, user_messages, prepended_conversation and AttackParameters.from_seed_group_async raise a ValidationError for those seeds.

This happens with real data. The PKU-SafeRLHF loader sets metadata=None for prompts that have no harm category. With the default settings, that's 15,827 of its 38,641 seeds.

Steps/Code to Reproduce

from pyrit.models import SeedGroup, SeedPrompt

group = SeedGroup(seeds=[SeedPrompt(value="How do I bake bread?", data_type="text", metadata=None)])
print(group.next_message)

Expected Results

The message is built with empty prompt_metadata, the same as for a prompt that has no metadata set.

Actual Results

pydantic_core._pydantic_core.ValidationError: 1 validation error for MessagePiece
prompt_metadata
  Input should be a valid dictionary [type=dict_type, input_value=None, input_type=NoneType]

Versions

• OS: macOS 26.4.1 (arm64)
• Python 3.13.16
• PyRIT 1.2.0.dev0, main (6a23196), editable install

I have a one-line fix (prompt.metadata or {} where the message pieces are built) with a test. Happy to open a PR.