Add warning message empty THEN clause

src/compiler/checker.ts

@@ -12227,6 +12227,11 @@ namespace ts {
 
             checkExpression(node.expression);
             checkSourceElement(node.thenStatement);
+
+            if (node.thenStatement.kind === SyntaxKind.EmptyStatement) {
+                error(node.thenStatement, Diagnostics.The_body_of_an_if_statement_cannot_be_the_empty_statement);
+            }
+
             checkSourceElement(node.elseStatement);
         }
 

src/compiler/diagnosticMessages.json

@@ -799,7 +799,11 @@
     "'=' can only be used in an object literal property inside a destructuring assignment.": {
         "category": "Error",
         "code": 1312
-    },    
+    },
+    "The body of an 'if' statement cannot be the empty statement.": {
+        "category": "Error",
+        "code": 1313
+    },
     "Duplicate identifier '{0}'.": {
         "category": "Error",
         "code": 2300