python3-grpcio: make sure that GRPC_CFLAGS is expanded to empty

* with bitbake change from:
  https://git.openembedded.org/bitbake/commit/?id=699e36c270d863258502d315ed00a1b940bfbf96
  the GRPC_CFLAGS now stays None when toolchain-clang isn't used
  causing:
  i686-oe-linux-gcc: error: ${GRPC_CFLAGS}: linker input file not found: No such file or directory

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-python/recipes-devtools/python/python3-grpcio_1.38.1.bb

@@ -34,6 +34,7 @@ BORING_SSL:arm = "1"
 BORING_SSL ?= "0"
 export GRPC_BUILD_WITH_BORING_SSL_ASM = "${BORING_SSL}"
 
+GRPC_CFLAGS ?= ""
 GRPC_CFLAGS:append:toolchain-clang = " -fvisibility=hidden -fno-wrapv -fno-exceptions"
 export GRPC_PYTHON_CFLAGS = "${GRPC_CFLAGS}"