Error when `capture_source` is enabled and `test_start` is `None`

[maxim-arcsec]

Hi, ran into and issue when enabling capture_source.

openhtf/openhtf/core/test_descriptor.py

Lines 326 to 327 in 618b914

	if CONF.capture_source:
	trigger.code_info = htf_test_record.CodeInfo.for_function(trigger.func)

When capture_source is enabled and no test_start is given, it tries to get the func attribute on a None value.