Fixed panic if skaffold.yaml is empty (#1216)

[varunkashyap]

The absence of apiVersion from skaffold.yaml hadn't been tested, a
nil err was being sent to errors.Wrapf which then returned nil as error
leading to wrong success of configuration parse operation.

We are now creating a new error instead of relying on wraping a
potentially nil err with errors.Wrapf.

[codecov-io]

Codecov Report

Merging #1221 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   41.41%   41.48%   +0.07%     
==========================================
  Files          95       95              
  Lines        4233     4233              
==========================================
+ Hits         1753     1756       +3     
+ Misses       2309     2307       -2     
+ Partials      171      170       -1

Impacted Files	Coverage Δ
pkg/skaffold/schema/versions.go	64.86% <100%> (+8.1%)	⬆️

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