Make get_app name optional

[ahopkins]

Resolves #2048

When only one Sanic instance has been registered, get_app() should return it even if no name has been passed.

Sanic("app")

app = Sanic.get_app()

[codecov]

Codecov Report

Merging #2053 (dce4e85) into master (400f54c) will increase coverage by 0.015%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master     #2053       +/-   ##
=============================================
+ Coverage   91.981%   91.995%   +0.015%     
=============================================
  Files           36        36               
  Lines         3292      3298        +6     
  Branches       559       562        +3     
=============================================
+ Hits          3028      3034        +6     
  Misses         179       179               
  Partials        85        85               

Impacted Files	Coverage Δ
sanic/app.py	91.489% <100.000%> (+0.110%)	⬆️

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