cstring.default.len raises a js error instead of returning 0 as in other backends

[timotheecour]

Example

proc main()=
  var a: cstring
  echo a.len
static: main()
main()

Current Output

c:
vm: 0
rt: 0
js:
vm: 0
rt: TypeError: Cannot read property 'length' of null

Expected Output

c:
vm: 0
rt: 0
js:
vm: 0
rt: 0

Additional Information

• 1.5.1 2c6f5ae
• (different issue) cstring("") != nil fails at CT; isNil sometimes returns false for nil cstring #15730

[metagn]

a.length => a != null ? a.length : 0 in jsgen with a temp should do it

[ringabout]

ditto: low/high(cstring(nil)) in JS backend