[ggj] fix(engx): Handle nulls with poll() in MockServiceImpl

[miraleung]

Improves the error message for client library users, instead of the more obscure error that previously occurred. In addition, remove() does not handle the empty case, while poll() does.

Before this change, here's the error one would get if responses was empty and responses.remove() was invoked.

com.google.cloud.datastream.v1alpha1.DatastreamClientTest > listLocationsTest STANDARD_ERROR
    Mar 10, 2021 5:26:18 PM io.grpc.internal.SerializingExecutor run
    SEVERE: Exception while executing runnable io.grpc.internal.ServerImpl$JumpToApplicationThreadServerStreamListener$1HalfClosed@4ecefb07
    java.util.NoSuchElementException
        at java.util.LinkedList.removeFirst(LinkedList.java:270)
        at java.util.LinkedList.remove(LinkedList.java:685)
        at com.google.cloud.location.MockLocationsImpl.listLocations(MockLocationsImpl.java:64)

[codecov]

Codecov Report

Merging #693 (84fe695) into master (edd7443) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #693   +/-   ##
=======================================
  Coverage   92.46%   92.47%           
=======================================
  Files         124      124           
  Lines       13344    13351    +7     
  Branches      952      952           
=======================================
+ Hits        12339    12346    +7     
  Misses        761      761           
  Partials      244      244           

Impacted Files	Coverage Δ
...r/gapic/composer/MockServiceImplClassComposer.java	99.76% <100.00%> (+<0.01%)	⬆️

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