[BUG] Null Pointer Exception in Key Value Processor

[kkondaka]

Describe the bug
A Null pointer exception occurs in Key Value Processor with the following stack trace

2023-12-29T11:03:28.730 [osi-logs-dev-pipeline-processor-worker-1-thread-2] ERROR org.opensearch.dataprepper.pipeline.ProcessWorker - Encountered exception during pipeline osi-logs-dev-pipeline processing
java.lang.NullPointerException: null
at java.base/java.util.regex.Matcher.getTextLength(Matcher.java:1770) ~[?:?]
at java.base/java.util.regex.Matcher.reset(Matcher.java:416) ~[?:?]
at java.base/java.util.regex.Matcher.(Matcher.java:253) ~[?:?]
at java.base/java.util.regex.Pattern.matcher(Pattern.java:1134) ~[?:?]
at java.base/java.util.regex.Pattern.split(Pattern.java:1262) ~[?:?]
at org.opensearch.dataprepper.plugins.processor.keyvalue.KeyValueProcessor.doExecute(KeyValueProcessor.java:239) ~[key-value-processor-2.6.0.jar:?]
at org.opensearch.dataprepper.model.processor.AbstractProcessor.lambda$execute$0(AbstractProcessor.java:54) ~[data-prepper-api-2.6.0.jar:?]
at io.micrometer.core.instrument.composite.CompositeTimer.record(CompositeTimer.java:69) ~[micrometer-core-1.11.3.jar:1.11.3]
at org.opensearch.dataprepper.model.processor.AbstractProcessor.execute(AbstractProcessor.java:54) ~[data-prepper-api-2.6.0.jar:?]
at org.opensearch.dataprepper.pipeline.ProcessWorker.doRun(ProcessWorker.java:133) ~[data-prepper-core-2.6.0.jar:?]
at org.opensearch.dataprepper.pipeline.ProcessWorker.run(ProcessWorker.java:61) ~[data-prepper-core-2.6.0.jar:?]
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515) ~[?:?]
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[?:?]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[?:?]
at java.base/java.lang.Thread.run(Thread.java:829) [?:?]

To Reproduce
To reproduce the issue, send input that does not have the "source" in the key value processor config.

Expected behavior
Input without the configured source should be ignored and the not crash data prepper.

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Ubuntu 20.04 LTS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.