Return empty list in bootstraper when discovery client disabled.

[ryanjbaxter]

Related to spring-cloud/spring-cloud-consul#785

[codecov]

Codecov Report

Merging #4099 (0328a66) into 3.1.x (9b6b0ab) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##              3.1.x    #4099      +/-   ##
============================================
+ Coverage     55.49%   55.53%   +0.03%     
- Complexity      524      528       +4     
============================================
  Files            58       58              
  Lines          2501     2503       +2     
  Branches        234      236       +2     
============================================
+ Hits           1388     1390       +2     
  Misses         1024     1024              
  Partials         89       89              

Impacted Files	Coverage Δ
.../eureka/config/EurekaConfigServerBootstrapper.java	82.60% <100.00%> (+1.65%)	⬆️

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