Python: empty-string instructions inject a contentless system message

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### Description

`prepend_instructions_to_messages(messages, "")` injects a system message whose content is an empty string. Only `None` short-circuits; an empty or whitespace-only instruction list still produces the contentless system message ahead of the real conversation.

Chat options commonly default `instructions` to `""`, so every request from an agent that never set instructions carries an empty system message into the model call.

### Reproduction

```python
from agent_framework import Message, prepend_instructions_to_messages

out = prepend_instructions_to_messages([Message(role="user", contents=["hi"])], "")
# result: [system(''), user('hi')] -- the empty system message is noise
```

### Expected behavior

An empty or all-whitespace instruction adds no message; a real instruction prepends verbatim (its own whitespace untouched).

### Environment

agent-framework python main (`b0437908`)

Start at the prepend_instructions_to_messages entry point and reproduce the empty-string case shown in the issue. Add coverage for empty and whitespace-only instructions, while confirming real instructions are prepended verbatim and no message is added for blank input.

Written by the indexing model from the issue text.