Added test cases for empty character scalars

Co-authored-by: Aman Godara <34789087+Aman-Godara@users.noreply.github.com>

Author: milancurcic

src/tests/string/test_string_operator.f90

@@ -103,9 +103,11 @@ subroutine test_concat
 
         a = "a"
         b = "b"
-        call check("a" //  b  == "ab")
-        call check( a  // "b" == "ab")
-        call check( a  //  b  == "ab")
+        call check( "a" //  b  == "ab" )
+        call check( a  // "b" == "ab" )
+        call check( a  //  b  == "ab" )
+        call check( a  //  ""  == "a" )
+        call check( ""  //  b  == "b" )
     end subroutine test_concat
 
 end module test_string_operator