fix getNullValue for cstring in VM, make other VM code aware of nil cstring (#22527)

* fix getNullValue for cstring in VM

fixes #22524

* very ugly fixes, but fix #15730

* nil cstring len works, more test lines

* fix high

(cherry picked from commit 942f846f04)

tests/vm/tvmmisc.nim

@@ -733,3 +733,34 @@ block: # bug #22190
     tab = mkOpTable(Berlin)
 
   doAssert not tab
+
+block: # issue #22524
+  const cnst = cstring(nil)
+  doAssert cnst.isNil
+  doAssert cnst == nil
+  let b = cnst
+  doAssert b.isNil
+  doAssert b == nil
+
+  let a = static: cstring(nil)
+  doAssert a.isNil
+
+  static:
+    var x: cstring
+    doAssert x.isNil
+    doAssert x == nil
+    doAssert x != ""
+
+block: # issue #15730
+  const s: cstring = ""
+  doAssert s != nil
+
+  static:
+    let s: cstring = ""
+    doAssert not s.isNil
+    doAssert s != nil
+    doAssert s == ""
+
+static: # more nil cstring issues
+  let x = cstring(nil)
+  doAssert x.len == 0