[generator] Add string cast to prevent CS1503.

Author: jpobst

tools/generator/Java.Interop.Tools.Generator.ObjectModel/Symbols/StringSymbol.cs

@@ -123,7 +123,7 @@ public string[] PreCall (CodeGenerationOptions opt, string var_name)
 				};
 			}
 			return new[]{
-				$"IntPtr {native_name} = JNIEnv.NewString ({managed_name});",
+				$"IntPtr {native_name} = JNIEnv.NewString ((string{opt.NullableOperator}){managed_name});",
 			};
 		}