[Feature] Checks for Nullable strings (#6050)

* Allows null strings in createString method c#

* Adds nullable strings to JS and swift

* Changes js checks

* Fixes typo

tests/FlatBuffers.Test.Swift/Tests/FlatBuffers.Test.SwiftTests/FlatBuffersTests.swift

@@ -60,6 +60,12 @@ final class FlatBuffersTests: XCTestCase {
         XCTAssertEqual(c.name, country)
     }
     
+    func testWriteNullableStrings() {
+        var b = FlatBufferBuilder()
+        XCTAssertTrue(b.create(string: nil).isEmpty)
+        XCTAssertTrue(b.createShared(string: nil).isEmpty)
+    }
+    
     func testWriteOptionalValues() {
         var b = FlatBufferBuilder()
         let root = optional_scalars_ScalarStuff.createScalarStuff(&b,
@@ -74,7 +80,6 @@ final class FlatBuffersTests: XCTestCase {
         XCTAssertEqual(scalarTable.defaultI8, 42)
         XCTAssertEqual(scalarTable.justU8, 100)
         XCTAssertEqual(scalarTable.maybeU8, 10)
-        
     }
 }
 

tests/FlatBuffers.Test/FlatBufferBuilderTests.cs

@@ -350,5 +350,14 @@ namespace FlatBuffers.Test
             var endOffset = fbb.Offset;
             Assert.AreEqual(endOffset, storedOffset);
         }
+
+        [FlatBuffersTestMethod]
+        public void FlatBufferBuilder_Add_null_String()
+        {
+            var fbb = new FlatBufferBuilder(16);
+            string s = null;
+            Assert.AreEqual(fbb.CreateSharedString(s).Value, 0);
+            Assert.AreEqual(fbb.CreateString(s).Value, 0);
+        }
     }
 }

tests/JavaScriptTest.js

@@ -50,7 +50,9 @@ function main() {
   test64bit();
   testUnicode();
   fuzzTest1();
-
+  testNullStrings();
+  testSharedStrings();
+  
   console.log('FlatBuffers test: completed successfully');
 }
 
@@ -437,4 +439,19 @@ function fuzzTest1() {
   }
 }
 
+function testSharedStrings() {
+  var shared_string = "Hello world";
+  var builder = new flatbuffers.Builder();
+  let mainOffset = builder.createSharedString(shared_string);
+  assert.strictEqual(builder.createSharedString(shared_string), mainOffset);
+}
+
+function testNullStrings() {
+  var builder = new flatbuffers.Builder();
+  assert.strictEqual(builder.createString(null), 0);
+  assert.strictEqual(builder.createSharedString(null), 0);
+  assert.strictEqual(builder.createString(undefined), 0);
+  assert.strictEqual(builder.createSharedString(undefined), 0);
+}
+
 main();