Fix NullPointerException in MigrateApiParamSchemaValue for argument-less @ApiParam

A marker @ApiParam / @parameter annotation written without parentheses has
null arguments (J.Annotation.getArguments() returns null, not an empty list).
visitAnnotation iterated the arguments without a null check, throwing a
NullPointerException and aborting the whole recipe run.

Guard against null arguments and return the annotation unchanged, since there
is nothing to migrate when no arguments are present.

Author: jsgv

src/main/java/org/openrewrite/openapi/swagger/MigrateApiParamSchemaValue.java

@@ -44,7 +44,7 @@ class MigrateApiParamSchemaValue extends JavaIsoVisitor<ExecutionContext> {
     @Override
     public Annotation visitAnnotation(Annotation annotation, ExecutionContext ctx) {
         J.Annotation a = super.visitAnnotation(annotation, ctx);
-        if (!PARAMETER_ANNOTATION_MATCHER.matches(a)) {
+        if (!PARAMETER_ANNOTATION_MATCHER.matches(a) || a.getArguments() == null) {
             return a;
         }
 

src/test/java/org/openrewrite/openapi/swagger/SwaggerToOpenAPITest.java

@@ -198,6 +198,56 @@ class Example {
         );
     }
 
+    @Test
+    void migrateApiParamWithoutArguments() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              import io.swagger.annotations.ApiParam;
+
+              class Example {
+                @ApiParam
+                private Integer foo;
+              }
+              """,
+            """
+              import io.swagger.v3.oas.annotations.Parameter;
+
+              class Example {
+                @Parameter
+                private Integer foo;
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void migrateApiParamWithoutArgumentsOnMethodParameter() {
+        rewriteRun(
+          //language=java
+          java(
+            """
+              import io.swagger.annotations.ApiParam;
+
+              class Example {
+                void create(@ApiParam Integer foo) {
+                }
+              }
+              """,
+            """
+              import io.swagger.v3.oas.annotations.Parameter;
+
+              class Example {
+                void create(@Parameter Integer foo) {
+                }
+              }
+              """
+          )
+        );
+    }
+
     /**
      *
      * Same test as {@link #migrateApiParam()} making sure the order of the annotation properties doesn't break the logic