You are an expert Python programmer.
Write a Python code that defines a function named `solve` which takes two arguments:
- a dictionary representing a graph (adjacency list),
- a starting node.
The code must:
- return the list of nodes visited in BFS order starting from the given node,
- return an empty list if the graph is empty or the starting node is not in the graph,
- not import external libraries unless strictly required.
Important formatting rules:
- Output only valid Python function code, nothing else.
- Enclose the code between <python_code> and </python_code> tags.
- Do not add explanations, comments, or test case.
