Fix potential NPEs in ParametersDiff and HtmlRender (#517)

Co-authored-by: Bhavesh Mistry <bhavesh@versa-networks.com>

Author: bmistry13

core/src/main/java/org/openapitools/openapidiff/core/compare/ParametersDiff.java

@@ -9,6 +9,8 @@
 import java.util.Optional;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+
+import org.apache.commons.lang3.StringUtils;
 import org.openapitools.openapidiff.core.model.Changed;
 import org.openapitools.openapidiff.core.model.ChangedParameters;
 import org.openapitools.openapidiff.core.model.DiffContext;
@@ -99,7 +101,7 @@ public boolean pathUnchangedParametersChanged(
       // Speedy Check. Use the map already created in changedParameters to check if missing param is
       // linked to newParam
       String newParameterName = context.getParameters().get(parameter.getName());
-      if (newParameterName.isEmpty()) return false;
+      if (StringUtils.isBlank(newParameterName)) return false;
 
       Optional<Parameter> newParameter =
           changedParameters.getIncreased().stream()

core/src/main/java/org/openapitools/openapidiff/core/output/HtmlRender.java

@@ -420,7 +420,7 @@ private LiTag li_changedParam(ChangedParameter changeParam) {
     Parameter leftParam = changeParam.getOldParameter();
     LiTag li = li().withText(changeParam.getName() + " in " + changeParam.getIn());
     if (changeRequired) {
-      li.withText(" change into " + (rightParam.getRequired() ? "required" : "not required"));
+      li.withText(" change into " + (rightParam.getRequired() != null &&  rightParam.getRequired() ? "required" : "not required"));
     }
     if (changeDescription) {
       li.withText(" Notes ")