fix: fix compatibility of CloudBigtableIO with the emulator env var (#3940)

There are 2 issues that prevented CloudBigtableIO from working with the emulator: 1. useBulk macro setting would override the endpoint to batch-bigtable.googleapis.com regardless if the BIGTABLE_EMULATOR_ENV was set 2. the emulator doesnt return sample row key sizes when when the data is tiny, so split calculations broke

Author: igorbernstein2

bigtable-client-core-parent/bigtable-hbase/src/main/java/com/google/cloud/bigtable/hbase/wrappers/veneer/BigtableHBaseVeneerSettings.java

@@ -429,6 +429,9 @@ private void configureConnection(StubSettings.Builder<?, ?> stubSettings, String
  endpointOverride =
  Optional.of(hostOverride.or(defaultHostname) + ":" + portOverride.or(defaultPort));
  } else if (endpointKey.equals(BIGTABLE_HOST_KEY)
+ // only apply batch endpoint when the default endpoint hasnt changed (ie. when
+ // BIGTABLE_EMULATOR_HOST is set)
+ && defaultEndpoint.equals("bigtable.googleapis.com:443")
  && configuration.getBoolean(BIGTABLE_USE_BATCH, false)) {
  endpointOverride = Optional.of(BIGTABLE_BATCH_DATA_HOST_DEFAULT + ":443");
  }

bigtable-dataflow-parent/bigtable-hbase-beam/src/main/java/com/google/cloud/bigtable/beam/CloudBigtableIO.java

@@ -335,7 +335,7 @@ protected long calculateEstimatedSizeBytes(PipelineOptions options) throws IOExc
  protected List<SourceWithKeys> split(
  long regionSize, long desiredBundleSizeBytes, byte[] startKey, byte[] stopKey)
  throws IOException {
- Preconditions.checkState(desiredBundleSizeBytes > 0);
+ Preconditions.checkState(desiredBundleSizeBytes >= 0);
  int splitCount = (int) Math.ceil((double) (regionSize) / (double) (desiredBundleSizeBytes));
 
  if (splitCount < 2 || stopKey.length == 0 || Bytes.compareTo(startKey, stopKey) >= 0) {
@@ -477,7 +477,8 @@ protected SourceWithKeys(CloudBigtableScanConfiguration configuration, long esti
  "Source keys not in order: [%s, %s]",
  Bytes.toStringBinary(startRow), Bytes.toStringBinary(stopRow)));
  }
- Preconditions.checkState(estimatedSize > 0, "Source size must be positive", estimatedSize);
+ Preconditions.checkState(
+ estimatedSize >= 0, "Source size cannot be negative", estimatedSize);
  }
  this.estimatedSize = estimatedSize;
  SOURCE_LOG.debug("Source with split: {}.", this);