network requests: do not access 1st byte of empty byte array

... because otherwise this would crash.
Apparently there are cases where the header name is empty.

Task-number: QTBUG-31667

(cherry picked from commit dd050d35d77e2f6ac37bd1266a843df55848022f)

Change-Id: I4c99094d2bad3ebb7cf44f83f9ed6a5f73b73016
Reviewed-by: Richard J. Moore <rich@kde.org>