[BEAM-11864] Use objects.equals instead of raw comparison

[Fokko]

The left hand side of the comparison can be equal as well.

When registering multiple coders, it will produce a NPE when checking if the fromRowFunction is already registered:

[WARNING] 
java.lang.NullPointerException
    at org.apache.beam.sdk.schemas.utils.AvroUtils$RowToGenericRecordFn.equals (AvroUtils.java:559)
    at org.apache.beam.sdk.schemas.SchemaCoder.equals (SchemaCoder.java:208)
    at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Objects.equal (Objects.java:52)
    at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.HashBiMap.seekByKey (HashBiMap.java:224)
    at org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.HashBiMap.get (HashBiMap.java:254)
    at org.apache.beam.runners.core.construction.SdkComponents.registerCoder (SdkComponents.java:265)
    at org.apache.beam.runners.core.construction.PCollectionTranslation.toProto (PCollectionTranslation.java:35)
    at org.apache.beam.runners.core.construction.SdkComponents.registerPCollection (SdkComponents.java:227)
    at org.apache.beam.runners.core.construction.PTransformTranslation.translateAppliedPTransform (PTransformTranslation.java:479)
    at org.apache.beam.runners.core.construction.PTransformTranslation$KnownTransformPayloadTranslator.translate (PTransformTranslation.java:424)
    at org.apache.beam.runners.core.construction.PTransformTranslation.toProto (PTransformTranslation.java:239)
    at org.apache.beam.runners.core.construction.SdkComponents.registerPTransform (SdkComponents.java:175)
    at org.apache.beam.runners.core.construction.PipelineTranslation$1.visitPrimitiveTransform (PipelineTranslation.java:87)
    at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit (TransformHierarchy.java:587)
    at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit (TransformHierarchy.java:579)
    at org.apache.beam.sdk.runners.TransformHierarchy$Node.access$500 (TransformHierarchy.java:239)
    at org.apache.beam.sdk.runners.TransformHierarchy.visit (TransformHierarchy.java:213)
    at org.apache.beam.sdk.Pipeline.traverseTopologically (Pipeline.java:468)
    at org.apache.beam.runners.core.construction.PipelineTranslation.toProto (PipelineTranslation.java:59)
    at org.apache.beam.runners.dataflow.DataflowRunner.run (DataflowRunner.java:933)
    at org.apache.beam.runners.dataflow.DataflowRunner.run (DataflowRunner.java:196)
    at org.apache.beam.sdk.Pipeline.run (Pipeline.java:322)
    at ....PubsubToBigQuery.main (PubsubToBigQuery.java:98)
    at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke (Method.java:498)
    at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:282)
    at java.lang.Thread.run (Thread.java:748)

This is because when you register a GenericRecord, the schema will be null, resulting in the NPE above.

Please add a meaningful description for your change here

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @iemejia).
• Format the pull request title like [BEAM-11864] Fixes bug in ApproximateQuantiles, where you replace BEAM-11864 with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

Post-Commit Tests Status (on master branch)

Lang	SDK	Dataflow	Flink	Samza	Spark	Twister2
Go		---		---		---
Java
Python				---		---
XLang				---		---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
Non-portable
Portable	---		---	---	---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[pabloem]

thanks! LGTM.