geekdroid-firebase: allow to validate QueryPagingSource boundary key
This is useful when ordering by @ServerTimestamp field. When the document
hasn't been sent to server it can't be used as a boundary key has the server
timestamp is null. This change allows client to make the check and avoid an
IllegalArgumentException.
syntax: glob
# generated files
bin/
gen/
doc/javadoc
# Local configuration file (sdk path, etc)
local.properties
# built application files
*.apk
*.ap_
# keystore
*.keystore
*.jks
# files for the dex VM
*.dex
# Java class files
*.class
# maven output folder
target
# Eclipse project files
.classpath
.project
.metadata
.settings
# IntelliJ files
.idea
*.iml
# OSX files
.DS_Store
# Windows files
Thumbs.db
# vi swap files
.*.sw?
# backup files
*.bak
*~
# maven eclipse files
.externalToolBuilders
maven-eclipse.xml
# gradle generated files
build/
.gradle/