PubSub Sink: Empty (non-null) message values bypass null check and break processing

#### Environment details

1. API: Pub/Sub Kafka Connector (Sink)
2. OS type and version: RHEL 8.10
3. Java version: Amazon Corretto 21.0.11.10.1 (build 21.0.11+10-LTS)
4. Connector version(s): 1.3.3

#### Steps to reproduce

1. Produce a Kafka message where:
- value = "" (empty string, not null)
- no headers/attributes are present
2. Allow the Pub/Sub Sink Connector to consume the message
3. Observe connector behaviour

#### Expected behaviour

Messages with no meaningful content (null OR empty string) should be treated as invalid and skipped.

The connector should continue processing subsequent messages without interruption.

#### Actual behaviour

When a message contains an empty string (`""`) rather than `null`, it bypasses the existing check:

if (attributes.size() == 0 && value == null)

This results in:
- The message being treated as valid
- Downstream processing issues
- Connector processing stopping or stalling

#### Code example

Current implementation:

```
if (attributes.size() == 0 && value == null) {
log.warn("Message received with no value and no attributes. Not publishing message");
SettableApiFuture nullMessageFuture = SettableApiFuture.create();
nullMessageFuture.set("No message");
addPendingMessageFuture(record.topic(), record.kafkaPartition(), nullMessageFuture);
continue;
}

```
#### Proposed fix

Update the condition to treat empty strings as equivalent to null:

```
--- src/main/java/com/google/pubsub/kafka/sink/CloudPubSubSinkTask.java
+++ src/main/java/com/google/pubsub/kafka/sink/CloudPubSubSinkTask.java
@@ -166,7 +166,7 @@
attributes.put(header.key(), header.value().toString());
}
}
- if (attributes.size() == 0 && value == null) {
+ if (attributes.isEmpty() && (value == null || value.isEmpty())) {
log.warn("Message received with no value and no attributes. Not publishing message");
SettableApiFuture nullMessageFuture = SettableApiFuture.create();
nullMessageFuture.set("No message");
```

#### Rationale

An empty string (`""`) is effectively equivalent to a missing payload in this context.

The current implementation only checks for `null`, allowing empty-but-invalid messages through, which can lead to inconsistent behaviour and connector stalls.

Handling both `null` and empty values consistently would improve robustness.

#### Stack trace

No stack trace available.

#### Additional information

This has been observed in environments where upstream producers occasionally emit empty string payloads instead of null values.

Start in src/main/java/com/google/pubsub/kafka/sink/CloudPubSubSinkTask.java at the message validation condition around line 166. Review how records with no attributes and null values are skipped, then verify that empty-string values follow the same path. Done means both null and empty values are skipped and subsequent messages continue processing.

Written by the indexing model from the issue text.