build(core): fix prodtest build from build-docker

build-docker.sh will unconditionally pass in a value for the
VENDOR_HEADER envvar. This is a problem if the user didn't specify one,
because the value inside the container is going to be empty string.

We make prodtest look at whether the value of VENDOR_HEADER is truthy
instead of a literal None. Which is a correct logic: if the user
deliberately sets the value to an empty string, the build will fail
anyway, so we ignore such value.

core/SConscript.prodtest

@@ -196,9 +196,9 @@ obj_program.extend(env.Object(source=SOURCE_HAL))
 MODEL_IDENTIFIER = tools.get_model_identifier(TREZOR_MODEL)
 
 
-if (vh := ARGUMENTS.get("VENDOR_HEADER", None)) is not None:
+if (vh := ARGUMENTS.get("VENDOR_HEADER", None)):
     VENDORHEADER = vh
-elif (vh := os.environ.get("VENDOR_HEADER", None)) is not None:
+elif (vh := os.environ.get("VENDOR_HEADER", None)):
     # TODO looking at envvars in a build script is not very nice. But justifiable in case
     # of vendor header which does not affect reproducibility of the build. Nonetheless,
     # we should figure out a cleaner way to pass in this argument, without having to teach