[BUG] Attempting to evaluate if a key is null throws an Exception if the value is a List<String> for conditional expressions

[graytaylor0]

Describe the bug
Evaluating a statement like

when: '/my_field == null'

will throw an Exception if the value of my_field does not match the expression ANTLR language

To Reproduce
Steps to reproduce the behavior:

1. Create a pipeline with a when condition equal to /my_field == null
2. Send an Event where my_field is an array, such as { "my_field": ["2024-02-09 19:26:25.308", "2024-02-09 19:26:25.308"] }
3. Observe the exception

2024-02-12T10:28:13,789 [test-pipeline-processor-worker-1-thread-1] ERROR org.opensearch.dataprepper.expression.ParseTreeEvaluator - Unable to evaluate event
org.opensearch.dataprepper.expression.ExpressionCoercionException: Unsupported type for value [2024-02-09 19:26:25.308, 2024-02-09 19:26:25.308]
	at org.opensearch.dataprepper.expression.ParseTreeCoercionService.lambda$new$0(ParseTreeCoercionService.java:33) ~[data-prepper-expression-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeCoercionService.resolveJsonPointerValue(ParseTreeCoercionService.java:103) ~[data-prepper-expression-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeCoercionService.coercePrimaryTerminalNode(ParseTreeCoercionService.java:69) ~[data-prepper-expression-2.7.0-SNAPSHOT.jar:?]
	at org.opensearch.dataprepper.expression.ParseTreeEvaluatorListener.visitTerminal(ParseTreeEvaluatorListener.java:69) ~[data-prepper-expression-2.7.0-SNAPSHOT.jar:?]
	at org.antlr.v4.runtime.tree.ParseTreeWalker.walk(ParseTreeWalker.java:29) ~[antlr4-runtime-4.10.1.jar:4.10.1]

Expected behavior
For checks of /my_field != null or /my_field == null, we should convert the value of my_field to an Object and check that it is null or not null, and should not rely on the ANTLR language for conditional expressions

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.