Fix empty dict literal parsing. (#3833)

It was errornously parsed as a set literal.

joern-cli/frontends/pysrc2cpg/pythonGrammar.jj

@@ -3556,25 +3556,34 @@ iexpr setOrDictOrSetCompOrDictComp(): {
   ArrayList<iexpr> valueExpressions = new ArrayList<iexpr>();
   ArrayList<Comprehension> comprehensions = null;
   // If isSet is false it is a dict.
-  boolean isSet = true;
+  boolean isSet = false;
 } {
   <CURLY_OPEN> { startToken = token; }
   (
     (
       (
-        dictKeyOrSetValueExpr = expression()
-      | dictKeyOrSetValueExpr = starredBitwiseOr()
-      | (<DOUBLE_STAR> doubleStarExpr = expression() { isSet = false; })
+        (
+          (dictKeyOrSetValueExpr = expression()
+          | dictKeyOrSetValueExpr = starredBitwiseOr()
+          )
+          (
+            (
+              <COLON> dictValueExpr = expression()
+              {
+                if (doubleStarExpr != null) {
+                  throw new ParseException("TODO");
+                }
+              }
+            )
+          | (
+              {
+                isSet = true;
+              }
+            )
+          )
+        )
+      | <DOUBLE_STAR> doubleStarExpr = expression()
       )
-      (
-        <COLON> dictValueExpr = expression()
-        {
-          if (doubleStarExpr != null) {
-            throw new ParseException("TODO");
-          }
-          isSet = false;
-        }
-      )?
       (
         (comprehensions = forIfClauses())
       | (

joern-cli/frontends/pysrc2cpg/src/test/scala/io/joern/pysrc2cpg/cpg/DictCpgTests.scala

@@ -0,0 +1,20 @@
+package io.joern.pysrc2cpg.cpg
+
+import io.joern.pysrc2cpg.Py2CpgTestContext
+import io.shiftleft.codepropertygraph.generated.DispatchTypes
+import io.shiftleft.semanticcpg.language.*
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+class DictCpgTests extends AnyWordSpec with Matchers {
+
+  "empty dict" should {
+    lazy val cpg = Py2CpgTestContext.buildCpg("""{}""".stripMargin)
+
+    "be represented as `dictLiteral`" in {
+      val callOption = cpg.call.methodFullName("<operator>.dictLiteral").nextOption()
+      callOption.isDefined shouldBe true
+      callOption.get.code shouldBe "{}"
+    }
+  }
+}

joern-cli/frontends/pysrc2cpg/src/test/scala/io/joern/pysrc2cpg/cpg/SetCpgTests.scala

@@ -0,0 +1,20 @@
+package io.joern.pysrc2cpg.cpg
+
+import io.joern.pysrc2cpg.Py2CpgTestContext
+import io.shiftleft.codepropertygraph.generated.DispatchTypes
+import io.shiftleft.semanticcpg.language.*
+import org.scalatest.matchers.should.Matchers
+import org.scalatest.wordspec.AnyWordSpec
+
+class SetCpgTests extends AnyWordSpec with Matchers {
+
+  "set" should {
+    lazy val cpg = Py2CpgTestContext.buildCpg("""{1}""".stripMargin)
+
+    "be represented as `setLiteral`" in {
+      val callOption = cpg.call.methodFullName("<operator>.setLiteral").nextOption()
+      callOption.isDefined shouldBe true
+      callOption.get.code shouldBe "{1}"
+    }
+  }
+}