Fix table name returned for CASTed columns

[rulle-io]

Problem

As reported by an internal user, when --sqlFile parameter is used and one of the columns is CASTed

SELECT
  CAST(date_test AS varchar) AS date_str,

dbeam throws an exception

[main] ERROR com.spotify.dbeam.jobs.ExceptionHandling 
- Failure: org.apache.avro.SchemaParseException: Empty name
at org.apache.avro.Schema.validateName(Schema.java:1525)
at org.apache.avro.Schema.access$400(Schema.java:87)
at org.apache.avro.Schema$Name.<init>(Schema.java:675)
at org.apache.avro.Schema.createRecord(Schema.java:212)
at org.apache.avro.SchemaBuilder$RecordBuilder.fields(SchemaBuilder.java:1805)
at com.spotify.dbeam.avro.JdbcAvroSchema.createAvroSchema(JdbcAvroSchema.java:113)
at com.spotify.dbeam.avro.JdbcAvroSchema.createSchemaByReadingOneRow(JdbcAvroSchema.java:86)
at com.spotify.dbeam.avro.BeamJdbcAvroSchema.generateAvroSchema(BeamJdbcAvroSchema.java:90)
at com.spotify.dbeam.avro.BeamJdbcAvroSchema.getAvroSchema(BeamJdbcAvroSchema.java:77)
at com.spotify.dbeam.avro.BeamJdbcAvroSchema.createSchema(BeamJdbcAvroSchema.java:54)
at com.spotify.dbeam.jobs.JdbcAvroJob.prepareExport(JdbcAvroJob.java:101)
at com.spotify.dbeam.jobs.JdbcAvroJob.runExport(JdbcAvroJob.java:145)
at com.spotify.dbeam.jobs.JdbcAvroJob.main(JdbcAvroJob.java:153)

It turned out that ResultSetMetaData.getTableName(...) returns an empty string for CASTed columns (on Postgres), which is not accepted later by org.apache.avro.Schema.

Changes

The change is to either return non-empty value or default one.

Tests:

• "Unit tests are included"
• "I successfully ran my test jobs with this code"

Checklist for PR author(s)

• Changes are covered by unit tests (no major decrease in code coverage %) and/or integration tests.
• Ensure code formating (use mvn com.coveo:fmt-maven-plugin:format org.codehaus.mojo:license-maven-plugin:update-file-header)
• Document any relevant additions/changes in the appropriate spot in javadocs/docs/README.

[codecov]

Codecov Report

Merging #352 (e070c9f) into master (7612a77) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #352      +/-   ##
============================================
+ Coverage     90.08%   90.34%   +0.25%     
- Complexity      230      233       +3     
============================================
  Files            25       25              
  Lines           908      911       +3     
  Branches         65       67       +2     
============================================
+ Hits            818      823       +5     
+ Misses           59       58       -1     
+ Partials         31       30       -1     

[rulle-io]

@labianchin