Repair nil check of compose()

[nanmu42]

An interface is nil only when both its type and value are nil. Detail: https://golang.org/doc/faq#nil_error

A big thank you to llya Evdokimov (@engenegr) for finding this bug (#3) ! ;)

[codecov[bot]]

Codecov Report

Merging #4 into master will decrease coverage by 0.1%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   78.43%   78.32%   -0.11%     
==========================================
  Files          10       10              
  Lines         255      263       +8     
==========================================
+ Hits          200      206       +6     
- Misses         33       34       +1     
- Partials       22       23       +1

Impacted Files	Coverage Δ
reflect.go	91.3% <0%> (ø)	:arrow_up:
helper.go	78.12% <100%> (+4.05%)	:arrow_up:
client.go	51.35% <33.33%> (-0.77%)	:arrow_down:

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