fixed null pointer exception in mojo

Author: Nithin Tatikonda

maven/src/main/java/org/openapitools/openapidiff/maven/OpenApiDiffMojo.java

@@ -1,5 +1,8 @@
 package org.openapitools.openapidiff.maven;
 
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
 import org.apache.maven.plugin.AbstractMojo;
 import org.apache.maven.plugin.MojoExecutionException;
 import org.apache.maven.plugin.MojoFailureException;
@@ -11,10 +14,6 @@
 import org.openapitools.openapidiff.core.output.ConsoleRender;
 import org.openapitools.openapidiff.core.output.HtmlRender;
 
-import java.io.File;
-import java.io.FileWriter;
-import java.io.IOException;
-
 /** A Maven Mojo that diffs two OpenAPI specifications and reports on differences. */
 @Mojo(name = "diff", defaultPhase = LifecyclePhase.TEST)
 public class OpenApiDiffMojo extends AbstractMojo {
@@ -31,10 +30,10 @@ public class OpenApiDiffMojo extends AbstractMojo {
   Boolean failOnChanged = false;
 
   @Parameter(property = "outputType")
-  String outputType;
+  String outputType = "";
 
   @Parameter(property = "outputFile")
-  String outputFile;
+  String outputFile = "";
 
   @Parameter(property = "skip", defaultValue = "false")
   Boolean skip = false;