Fix printing and comparing uninitialized strings

tests/js/taddnilstr.nim

@@ -1,4 +0,0 @@
-var x = "foo".cstring
-var y: string
-add(y, x)
-doAssert y == "foo"

tests/js/tnilstrs.nim

@@ -0,0 +1,17 @@
+block:
+  var x: string
+  var y = "foo"
+
+  echo x
+  doAssert x == ""
+  doAssert "" == x
+
+  add(x, y)
+  y[0] = 'm'
+  doAssert y == "moo" and x == "foo"
+
+block:
+  var x = "foo".cstring
+  var y: string
+  add(y, x)
+  doAssert y == "foo"

tests/js/tstrconcat.nim

@@ -1,5 +0,0 @@
-var x: string
-var y = "foo"
-add(x, y)
-y[0] = 'm'
-doAssert y == "moo" and x == "foo"