Handle empty service config better

[loosebazooka]

fixes the error message for non appengine-web.xml projects when not specifying a service.

#421 (comment)

[codecov]

Codecov Report

Merging #423 into master will increase coverage by 0.55%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #423      +/-   ##
============================================
+ Coverage     58.94%   59.50%   +0.55%     
- Complexity      117      120       +3     
============================================
  Files            32       32              
  Lines           643      642       -1     
  Branches         49       50       +1     
============================================
+ Hits            379      382       +3     
+ Misses          247      244       -3     
+ Partials         17       16       -1     

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/tools/maven/run/AbstractRunMojo.java	42.85% <100.00%> (+34.52%)	4.00 <3.00> (+3.00)
.../java/com/google/cloud/tools/maven/run/Runner.java	75.43% <100.00%> (+0.43%)	10.00 <0.00> (ø)

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[chanseokoh]

There's a code format issue.