Fix the issue where the service name is not checked for null when adding to the request header

[hanbingleixue]

What happened?

Before Spring is completely started, if a service is invoked but the service name is not initialized, an exception occurs when the request header is added.

How can we reproduce it (as minimally and precisely as possible)?

Before Spring is completely started, initiate a Feign request to reproduce the problem.

Anything else we need to know?

nothing

Sermant version

2.1.0

OS version

# On Linux:
$ cat /etc/os-release
# paste output here
$ uname -a
# paste output here

# On Windows:
C:\> wmic os get Caption, Version, BuildNumber, OSArchitecture
# paste output here