optimize: add nil check for MethodInfo which get from ServiceInfo in client.Call to ignore panic

[YangruiEmma]

What type of PR is this?

optimize

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• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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optimize: 在 client.Call 对 MethodInfo 增加 nil 检查，避免 panic

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en: Normally, there will be no panic, but there are abnormal situations. Adding nil-check is convenient for troubleshooting
zh(optional): 正常不会出现 panic，但存在异常情况，增加 nil 检查便于问题排查

(Optional) Which issue(s) this PR fixes:

(optional) The PR that updates user documentation:

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10 🎉

Comparison is base (ab3c244) 68.01% compared to head (103430a) 68.12%.

❗ Current head 103430a differs from pull request most recent head 18c99d5. Consider uploading reports for the commit 18c99d5 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1020      +/-   ##
===========================================
+ Coverage    68.01%   68.12%   +0.10%     
===========================================
  Files          248      248              
  Lines        18802    18810       +8     
===========================================
+ Hits         12789    12815      +26     
+ Misses        4845     4834      -11     
+ Partials      1168     1161       -7     

Impacted Files	Coverage Δ
client/client.go	76.67% <100.00%> (+0.11%)	⬆️
pkg/loadbalance/weighted_round_robin.go	94.28% <100.00%> (+0.53%)	⬆️

... and 4 files with indirect coverage changes

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