fix(core): Update QueryPagination page field to default to 0

[cshfang]

Issue #, if available: #842

Description of changes: This PR updates the default constructor for QueryPaginations to supply a default value of 0 for the page field. The Kotlin version of the QueryPaginationBuilder checks to see if the serialized map passed to it contains the page key, but since the serializeAsMap() method always adds the page key, a NullPointerException can arise as a result.

Another fix considered was to update the serializeAsMap() method to only add the page key if page is defined. This would be a reasonable fix but only when coupled with the current QueryPaginationBuilder utilities. Opting for the default value would not require the "is the page key defined check.

• Also of note - Rather than removing these checks from the utilities, opting to keep them there as they are, in theory, directly usable outside of Flutter method channel context as well.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #1533 (8aaa54e) into main (ce9c4fa) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #1533   +/-   ##
=======================================
  Coverage   46.47%   46.47%           
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  Files         262      262           
  Lines       10194    10194           
=======================================
  Hits         4738     4738           
  Misses       5456     5456           

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	36.63% <0.00%> (ø)
ios-unit-tests	85.12% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ify_core/lib/src/types/query/query_pagination.dart	0.00% <0.00%> (ø)

[dnys1]

Thanks!