fix the version string checks in KafkaIO

[liferoad]

cannot use Strings.isNullOrEmpty because of

> Compilation failed; see the compiler output below.
  /Users/runner/work/beam/beam/sdks/java/io/kafka/src/main/java/org/apache/beam/sdk/io/kafka/KafkaIO.java:1863: error: [argument] incompatible argument for parameter firstVersion of TransformUpgrader.compareVersions.
                && TransformUpgrader.compareVersions(requestedVersionString, "2.66.0") < 0) {
                                                     ^
    found   : @Initialized @Nullable String
    required: @Initialized @NonNull String
  2 errors

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[chamikaramj]

Thanks. LGTM.

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