Common: fix PathNotFoundException in PII enrichment

[chuwy]

This line:

enrich/modules/common/src/main/scala/com.snowplowanalytics.snowplow.enrich/common/enrichments/registry/pii/PiiPseudonymizerEnrichment.scala

Lines 273 to 276 in 85ce299

	val documentContext2 = documentContext.map(
	jsonPath,
	new ScrambleMapFunction(strategy, modifiedFields, fieldMutator.fieldName, jsonPath, schema)
	)

can throw following exception:

com.jayway.jsonpath.PathNotFoundException: Expected to find an object with property ['a','b','c','d','e'] in path $['properties']['foo'] but found 'null'. This is not a json object according to the JsonProvider: 'com.jayway.jsonpath.spi.json.JacksonJsonNodeJsonProvider'.
    at com.jayway.jsonpath.internal.path.PropertyPathToken.evaluate(PropertyPathToken.java:71)
    at com.jayway.jsonpath.internal.path.PathToken.handleObjectProperty(PathToken.java:81)
    at com.jayway.jsonpath.internal.path.PropertyPathToken.evaluate(PropertyPathToken.java:79)
    at com.jayway.jsonpath.internal.path.PathToken.handleObjectProperty(PathToken.java:81)
    at com.jayway.jsonpath.internal.path.PropertyPathToken.evaluate(PropertyPathToken.java:79)
    at com.jayway.jsonpath.internal.path.RootPathToken.evaluate(RootPathToken.java:62)
    at com.jayway.jsonpath.internal.path.CompiledPath.evaluate(CompiledPath.java:53)
    at com.jayway.jsonpath.JsonPath.map(JsonPath.java:244)
    at com.jayway.jsonpath.internal.JsonContext.map(JsonContext.java:161)
    at com.jayway.jsonpath.internal.JsonContext.map(JsonContext.java:155)
    at com.snowplowanalytics.snowplow.enrich.common.enrichments.registry.pii.PiiJson.jsonPathReplace(PiiPseudonymizerEnrichment.scala:275)
    at com.snowplowanalytics.snowplow.enrich.common.enrichments.registry.pii.PiiJson.$anonfun$modifyObjectIfSchemaMatches$6(PiiPseudonymizerEnrichment.scala:255)
    at scala.Option.map(Option.scala:230)

[benjben]

To add more details, this is happening when we want to hash foo.a, but foo (whose type is ["object", "null"]) is null