HoodieMetadataTableValidator fails if there is no completed instant for metadata table

{code:java}
23/01/15 12:08:33 WARN HoodieMetadataTableValidator: Metadata table is not available to read for now,
org.apache.hudi.exception.HoodieValidationException: There is no completed instant for metadata table.
at org.apache.hudi.utilities.HoodieMetadataTableValidator.checkMetadataTableIsAvailable(HoodieMetadataTableValidator.java:493)
at org.apache.hudi.utilities.HoodieMetadataTableValidator.doMetadataTableValidation(HoodieMetadataTableValidator.java:407)
at org.apache.hudi.utilities.HoodieMetadataTableValidator.doHoodieMetadataTableValidationOnce(HoodieMetadataTableValidator.java:380)
at org.apache.hudi.utilities.HoodieMetadataTableValidator.run(HoodieMetadataTableValidator.java:366)
at org.apache.hudi.utilities.HoodieMetadataTableValidator.main(HoodieMetadataTableValidator.java:350)
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.apache.spark.deploy.JavaMainApplication.start(SparkApplication.scala:52)
at org.apache.spark.deploy.SparkSubmit.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:958)
at org.apache.spark.deploy.SparkSubmit.doRunMain$1(SparkSubmit.scala:180)
at org.apache.spark.deploy.SparkSubmit.submit(SparkSubmit.scala:203)
at org.apache.spark.deploy.SparkSubmit.doSubmit(SparkSubmit.scala:90)
at org.apache.spark.deploy.SparkSubmit$$anon$2.doSubmit(SparkSubmit.scala:1046)
at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:1055)
at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala) {code}

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-5564
- Type: Bug

Start with HoodieMetadataTableValidator.java, especially checkMetadataTableIsAvailable at line 493 and its callers doMetadataTableValidation and doMetadataTableValidationOnce. Reproduce the case where the metadata table has no completed instant, then determine and verify the validator's expected behavior for that state.

Written by the indexing model from the issue text.