Better fix for IDEA-184723 Data flow algorithm complexity

"x==null" & "x!=null" facts are merged into "x is NULLABLE"

java/java-tests/testData/inspection/dataFlow/fixture/NullabilityBasics.java

@@ -0,0 +1,31 @@
+import org.jetbrains.annotations.*;
+
+class NullabilityBasics {
+
+  void test2() {
+    String x = getNullable();
+    if (x == null) {
+      System.out.println("x is null!");
+    }
+    if (isEmpty(x) && Math.random() > 0.5) {
+      return;
+    }
+    System.out.println(x.<warning descr="Method invocation 'trim' may produce 'java.lang.NullPointerException'">trim</warning>());
+  }
+
+  @Nullable String getNullable() {
+    return Math.random() > 0.5 ? null : "";
+  }
+
+  @Contract(value = "null -> true",pure = true)
+  static boolean isEmpty(@Nullable String s) {
+    return s == null || s.isEmpty();
+  }
+
+  void test(String x) {
+    if (x == null) {
+      System.out.println("x is null!");
+    }
+    System.out.println(x.<warning descr="Method invocation 'trim' may produce 'java.lang.NullPointerException'">trim</warning>());
+  }
+}