Return empty string for missing input

Related to GH-4.

Closes GH-5.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

Author: Rokt33r

Diff for: index.js

@@ -7,7 +7,7 @@ module.exports = stringify
 function stringify(value) {
   /* Nothing. */
   if (!value || typeof value !== 'object') {
-    return null
+    return ''
   }
 
   /* Node. */
@@ -26,7 +26,7 @@ function stringify(value) {
   }
 
   /* ? */
-  return null
+  return ''
 }
 
 function point(point) {

Diff for: readme.md

@@ -56,7 +56,7 @@ a node’s position.
 `string?` — A range `ls:cs-le:ce` (when given `node` or
 `position`) or a point `l:c` (when given `point`), where `l` stands
 for line, `c` for column, `s` for `start`, and `e` for
-end.  `null` is returned if the given value is neither `node`,
+end.  Empty string (`''`), is returned if the given value is neither `node`,
 `position`, nor `point`.
 
 ## Contribute

Diff for: test.js

@@ -4,11 +4,11 @@ var test = require('tape')
 var stringify = require('.')
 
 test('stringifyPosition', function(t) {
-  t.equal(stringify(), null, 'should return `null` with `undefined`')
-  t.equal(stringify(null), null, 'should return `null` with `null`')
-  t.equal(stringify('foo'), null, 'should return `null` with `string`')
-  t.equal(stringify(5), null, 'should return `null` with `number`')
-  t.equal(stringify({}), null, 'should return `null` with `{}`')
+  t.equal(stringify(), '', 'should return empty `string` with `undefined`')
+  t.equal(stringify(null), '', 'should return empty `string` with `null`')
+  t.equal(stringify('foo'), '', 'should return empty `string` with `string`')
+  t.equal(stringify(5), '', 'should return empty `string` with `number`')
+  t.equal(stringify({}), '', 'should return empty `string` with `{}`')
 
   t.equal(
     stringify({type: 'text'}),