[BUG] Key value processor will throw NPE if source key does not exist in the Event

[graytaylor0]

Describe the bug
The key value processor can throw NPE here (

data-prepper/data-prepper-plugins/key-value-processor/src/main/java/org/opensearch/dataprepper/plugins/processor/keyvalue/KeyValueProcessor.java

Line 239 in b4b4a98

final String[] groups = fieldDelimiterPattern.split(groupsRaw, 0);

) if the source key in the Event does not exist.

Expected behavior
Utilize the tags_on_failure to tag events that don't have the key, but otherwise will be a no-op and be flushed downstream.

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.

[dlvenable]

Duplicate of #3928.