Add null check for tables stats collector when collecting table policies

[chenselena]

Summary

This PR fixes the issue where tables that had a sharing policy set without a retention policy set would throw a null pointer exception when trying to collect retention stats.

java.lang.NullPointerException
	at com.linkedin.openhouse.jobs.util.TableStatsCollectorUtil.addEntriesToMap(TableStatsCollectorUtil.java:271)
	at com.linkedin.openhouse.jobs.util.TableStatsCollectorUtil.getTablePolicies(TableStatsCollectorUtil.java:250)
	at com.linkedin.openhouse.jobs.util.TableStatsCollectorUtil.populateTableMetadata(TableStatsCollectorUtil.java:155)
	at com.linkedin.openhouse.jobs.util.TableStatsCollector.collectTableStats(TableStatsCollector.java:25)
	at com.linkedin.openhouse.jobs.spark.Operations.collectTableStats(Operations.java:445)
	at com.linkedin.openhouse.jobs.spark.OperationsTest.testCollectTablePolicyStats(OperationsTest.java:603)

A null check is added to getTablePolicies so if sharing or retention policies do not exist on the table it will not collect those stats.

Changes

• Client-facing API Changes
• Internal API Changes
• Bug Fixes
• New Features
• Performance Improvements
• Code Style
• Refactoring
• Documentation
• Tests

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Testing Done

• Manually Tested on local docker setup. Please include commands ran, and their output.
• Added new tests for the changes made.
• Updated existing tests to reflect the changes made.
• No tests added or updated. Please explain why. If unsure, please feel free to ask for help.
• Some other form of testing like staging or soak time in production. Please explain.

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Added unit tests for testing stat collection on tables without a retention policy.

Additional Information

• Breaking Changes
• Deprecations
• Large PR broken into smaller PRs, and PR plan linked in the description.

For all the boxes checked, include additional details of the changes made in this pull request.