[backport:1.4] JS cstring null fixes (#16979)

* [backport:1.4] JS cstring null fixes
* fix JS move string
* make it look cleaner

(cherry picked from commit 81533a0014)

lib/system.nim

@@ -1957,8 +1957,14 @@ type
     when NimStackTraceMsgs:
       frameMsgLen*: int   ## end position in frameMsgBuf for this frame.
 
-when defined(js):
+when defined(js) or defined(nimdoc):
   proc add*(x: var string, y: cstring) {.asmNoStackFrame.} =
+    ## Appends `y` to `x` in place.
+    runnableExamples:
+      var tmp = ""
+      tmp.add(cstring("ab"))
+      tmp.add(cstring("cd"))
+      doAssert tmp == "abcd"
     asm """
       if (`x` === null) { `x` = []; }
       var off = `x`.length;
@@ -1967,7 +1973,15 @@ when defined(js):
         `x`[off+i] = `y`.charCodeAt(i);
       }
     """
-  proc add*(x: var cstring, y: cstring) {.magic: "AppendStrStr".}
+  proc add*(x: var cstring, y: cstring) {.magic: "AppendStrStr".} =
+    ## Appends `y` to `x` in place.
+    ## Only implemented for JS backend.
+    runnableExamples:
+      when defined(js):
+        var tmp: cstring = ""
+        tmp.add(cstring("ab"))
+        tmp.add(cstring("cd"))
+        doAssert tmp == cstring("abcd")
 
 elif hasAlloc:
   {.push stackTrace: off, profiler: off.}

lib/system/jssys.nim

@@ -634,7 +634,7 @@ proc genericReset(x: JSRef, ti: PNimType): JSRef {.compilerproc.} =
       asm "`result` = {m_type: `ti`};"
     else:
       asm "`result` = {};"
-  of tySequence, tyOpenArray:
+  of tySequence, tyOpenArray, tyString:
     asm """
       `result` = [];
     """