pc_pos_map for small methods is empty

[bout3fiddy]

Version Information

• vyper Version (output of vyper --version): 0.3.7
• OS: osx
• Python Version (output of python --version): 3.10.4

Bug

(vyper) ~/vyper $ cat tmp/baz.vy 

@external
def foo():
    pass

(vyper) ~/vyper $ vyc -f source_map tmp/baz.vy 

{"breakpoints": [], "error_map": {"51": "fallback function"}, "pc_breakpoints": [], "pc_jump_map": {"0": "-", "7": "-", "11": "-", "12": "-", "23": "-", "34": "-", "42": "-", "44": "-", "46": "-", "52": "-"}, "pc_pos_map": {}, "pc_pos_map_compressed": "-1:-1:0:-;;;;:::-;;:::-;:::-;;;;;;;:::-;;;;;:::-;;;;;:::-;;:::-;;:::-;;;;:::-;;;"}

pc_pos_map should not be empty.