Merged AssertThatObjectIsNull and AssertThatObjectIsNotNull to AssertThatObjectIsNullOrNotNull.

Extended tests to be more explicit regarding the expected quick fix messages.

src/test/resources/inspections/ObjectIsNullOrNotNull/ObjectIsNullOrNotNullBefore.java

@@ -0,0 +1,14 @@
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class ObjectIsNull {
+
+    private void objectIsNull() {
+        assertThat("").isEqualTo(null);
+        assertThat("").as("nah").isEqualTo(null);
+        assertThat(new Object()).isEqualTo(null);
+
+        assertThat("").isNotEqualTo(null);
+        assertThat("").as("nah").isNotEqualTo(null);
+        assertThat(new Object()).isNotEqualTo(null);
+    }
+}