NIFI-7264 Make jsonPath Expression Logging More Reasonable

add special handling of PathNotFoundExceptions to log to debug
fix spelling error
wrap debug log in guard per review

This closes apache#4148

Signed-off-by: Mike Thomsen <mthomsen@apache.org>

nifi-commons/nifi-expression-language/src/main/java/org/apache/nifi/attribute/expression/language/evaluation/functions/JsonPathEvaluator.java

@@ -16,6 +16,7 @@
  */
 package org.apache.nifi.attribute.expression.language.evaluation.functions;
 
+import com.jayway.jsonpath.PathNotFoundException;
 import org.apache.nifi.attribute.expression.language.EvaluationContext;
 import org.apache.nifi.attribute.expression.language.evaluation.Evaluator;
 import org.apache.nifi.attribute.expression.language.evaluation.QueryResult;
@@ -46,7 +47,16 @@ public QueryResult<String> evaluate(EvaluationContext context) {
         Object result = null;
         try {
             result = documentContext.read(compiledJsonPath);
+        } catch (PathNotFoundException pnf) {
+            // it is valid for a path not to be found, keys may not be there
+            // do not spam the error log for this, instead we can log debug if enabled
+            if (LOGGER.isDebugEnabled()) {
+                LOGGER.debug("PathNotFoundException for JsonPath " + compiledJsonPath.getPath(), pnf);
+            }
+            return EMPTY_RESULT;
         } catch (Exception e) {
+            // a failure for something *other* than path not found however, should at least be
+            // logged.
             LOGGER.error("Exception while reading JsonPath " + compiledJsonPath.getPath(), e);
             return EMPTY_RESULT;
         }