Fix IndexError when codegen is missing expected python_requirement targets

[Eric-Arellano]

This code could result in calling softwrap(""), which causes an exception unnecessarily.

pants/src/python/pants/backend/codegen/utils.py

Lines 46 to 57 in 9a6e8d3

	resolve_note = softwrap(
	(
	f"""
	Note that because `[python].enable_resolves` is set, you must specifically have a
	`python_requirement` target that uses the same resolve '{resolve}' as the target
	{codegen_address}. Alternatively, update {codegen_address} to use a different
	resolve.
	"""
	)
	if resolves_enabled
	else ""
	)

[ci skip-rust]
[ci skip-build-wheels]

[Eric-Arellano]

Note that this is only on Pants 2.12 because 2.13 and 2.14 already had this change applied in an unrelated PR that we can't cherry-pick.