Prevent NPE in Enum equals with empty select

Fixes #143

Author: timtebeek

src/main/java/org/openrewrite/staticanalysis/CompareEnumsWithEqualityOperator.java

@@ -55,7 +55,7 @@ public TreeVisitor<?, ExecutionContext> getVisitor() {
             @Override
             public J visitMethodInvocation(J.MethodInvocation method, ExecutionContext executionContext) {
                 J.MethodInvocation m = (J.MethodInvocation) super.visitMethodInvocation(method, executionContext);
-                if (enumEquals.matches(m)) {
+                if (enumEquals.matches(m) && m.getSelect() != null) {
                     Cursor parent = getCursor().dropParentUntil(is -> is instanceof J.Unary || is instanceof J.Block);
                     boolean isNot = parent.getValue() instanceof J.Unary && ((J.Unary) parent.getValue()).getOperator() == J.Unary.Type.Not;
                     if (isNot) {

src/test/java/org/openrewrite/staticanalysis/CompareEnumsWithEqualityOperatorTest.java

@@ -29,6 +29,7 @@ public void defaults(RecipeSpec spec) {
         spec.recipe(new CompareEnumsWithEqualityOperator());
     }
 
+    //language=java
     SourceSpecs enumA = java(
       """
         package a;
@@ -198,4 +199,21 @@ void m(Object parameterValue, Type partType) {
               """
           ));
     }
+
+    @Test
+    @Issue("https://github.com/openrewrite/rewrite-static-analysis/issues/143")
+    void noSelect() {
+        rewriteRun(
+          //language=java
+          java("""
+            package a;
+            public enum A {
+                FOO, BAR, BUZ;
+                boolean isFoo() {
+                    return equals(FOO);
+                }
+            }
+            """)
+        );
+    }
 }