[bug] Append `self.source_folder` into `Environment()`: TypeError: sequence item 0: expected str instance, NoneType found

[SpaceIm]

Environment Details (include every applicable attribute)

• Operating System+version: macOS Monterey
• Compiler+version: AppleClang 13
• Conan version: 1.52.0
• Python version: 3.10.6

Steps to reproduce (Include if Applicable)

run conan create on a recipe with this logic in generate()

from conan.tools.cmake import cmake_layout
from conan.tools.env import Environment

[...]

def layout(self):
    cmake_layout(self)

def generate(self):
    env = Environment()
    env.prepend_path("PKG_CONFIG_PATH", self.source_folder)
    env.vars(self).save_script("conanbuildenv_pkg_config_path")

Actually I've tested this block in a test_package (for conan v2 migration of pkgconf recipe in CCI)

Logs (Executed commands with output) (Include/Attach if Applicable)

ERROR: pkgconf/1.7.4 (test package): Error in generate() method, line 43
	envvars.save_script("conanbuildenv_pkg_config_path")
	TypeError: sequence item 0: expected str instance, NoneType found