Error when blt_add_code_checks calls blt_add_yapf_target about blt

[white238]

I see two solutions:

1. Have the user required to not provide a YAPF config file if you don't provide Python files to add code checks for (this seems less than robust).

2. Check on each of these if blocks that you are actually providing source files to run checks for:
   

   blt/cmake/SetupCodeChecks.cmake

   Lines 238 to 255 in 92450b0

   	if (YAPF_FOUND AND DEFINED arg_YAPF_CFG_FILE)
   	set(_check_target_name ${arg_PREFIX}_yapf_check)
   	blt_error_if_target_exists(${_check_target_name} ${_error_msg})
   	set(_style_target_name ${arg_PREFIX}_yapf_style)
   	blt_error_if_target_exists(${_style_target_name} ${_error_msg})
   	blt_add_yapf_target( NAME ${_check_target_name}
   	MODIFY_FILES FALSE
   	CFG_FILE ${arg_YAPF_CFG_FILE}
   	WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
   	SRC_FILES ${_py_sources} )
   	blt_add_yapf_target( NAME ${_style_target_name}
   	MODIFY_FILES TRUE
   	CFG_FILE ${arg_YAPF_CFG_FILE}
   	WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
   	SRC_FILES ${_py_sources} )
   	endif()

I would lean towards the second option.