bitbake: codeparser: Allow empty functions

The main parser and other code copes with empty python functions but
the python codeparser does not. Fix this to avoid errors with code like:

python do_build () {
}

which currently results in the obtuse:
"Failure expanding variable do_build: IndexError: string index out of range"

[YOCTO #7829]

(Bitbake rev: e4f594c670189e04d58ce7d160fc1d86123620af)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/codeparser.py

@@ -240,6 +240,9 @@ class PythonParser():
         self.unhandled_message = "while parsing %s, %s" % (name, self.unhandled_message)
 
     def parse_python(self, node):
+        if not node or not node.strip():
+            return
+
         h = hash(str(node))
 
         if h in codeparsercache.pythoncache: