fix: fix error when forwarding tff files

[Chever-John]

fix issue #50

Just add a judgment statement.

[Chever-John]

@spacewander @shuaijinchao Plz give me a approval :)

[shuaijinchao]

Add test cases?

[codecov]

Codecov Report

Merging #51 (c5d0d5f) into master (3de14f8) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head c5d0d5f differs from pull request most recent head 1a0ccde. Consider uploading reports for the commit 1a0ccde to get more accurate results

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   91.72%   91.77%   +0.05%     
==========================================
  Files          11       11              
  Lines         749      754       +5     
==========================================
+ Hits          687      692       +5     
  Misses         62       62              

Impacted Files	Coverage Δ
apisix/runner/utils/common.py	98.65% <100.00%> (+0.04%)	⬆️

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[Chever-John]

Add test cases?

Yeah~
I will do it!