fix: NPE issue with testMultipleRuns (#2050)

Added a `null` check for `firstPage.getSchema()` as we might get null schema when the job is long running (We get schema using dryRun in such cases) Fixes #2049 ☕️

Author: prash-mi

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/ConnectionImpl.java

@@ -234,8 +234,10 @@ public BigQueryResult executeSelect(
  BigQueryResult getResultSet(
  GetQueryResultsResponse firstPage, JobId jobId, String sql, Boolean hasQueryParameters) {
  if (firstPage.getJobComplete()
- && firstPage.getTotalRows()
- != null) { // firstPage.getTotalRows() is null if job is not complete
+ && firstPage.getTotalRows() != null
+ && firstPage.getSchema()
+ != null) { // firstPage.getTotalRows() is null if job is not complete. We need to make
+ // sure that the schema is not null, as it is required for the ResultSet
  return getSubsequentQueryResultsWithJob(
  firstPage.getTotalRows().longValue(),
  (long) firstPage.getRows().size(),