Check for null in VersionComparisonAnnotation

[dotasek]

Fixes a NullPointerException resulting from this call, which explicitly sets it as null. This was causing failures in IG Publisher in publishing ACME-FSH-IG-Example

org.hl7.fhir.core/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/renderers/StructureDefinitionRenderer.java

Line 4348 in 4766cb3

return compareString(Utilities.escapeXml(newMap), null, null, "mapping", d, Utilities.escapeXml(oldMap), null, mode);

See this stack trace.

render:94, VersionComparisonAnnotation (org.hl7.fhir.r5.comparison)
compareString:3372, StructureDefinitionRenderer (org.hl7.fhir.r5.renderers)
getMapping:4348, StructureDefinitionRenderer (org.hl7.fhir.r5.renderers)
generateElementInner:3564, StructureDefinitionRenderer (org.hl7.fhir.r5.renderers)
renderDict:3119, StructureDefinitionRenderer (org.hl7.fhir.r5.renderers)
dict:1049, StructureDefinitionRenderer (org.hl7.fhir.igtools.renderers)
generateOutputsStructureDefinition:11030, Publisher (org.hl7.fhir.igtools.publisher)
generateResourceHtml:9982, Publisher (org.hl7.fhir.igtools.publisher)
generateHtmlOutputs:9834, Publisher (org.hl7.fhir.igtools.publisher)
generate:7066, Publisher (org.hl7.fhir.igtools.publisher)
createIg:1052, Publisher (org.hl7.fhir.igtools.publisher)
execute:882, Publisher (org.hl7.fhir.igtools.publisher)
main:11921, Publisher (org.hl7.fhir.igtools.publisher)

[grahamegrieve]

made redundant by another commit