Let '' be the default carpet axes title [882]

- Reason: having no default at all for `title.text` would lead to a
  non-existing `title` container (through a call to
  `relinkPrivateKeys` deep down the stack) eventually and as consequence
  would require to safeguard access to `[ab]axis.title.text` by checking
  `[abaxis.title]` is defined before.

Author: rmoestl

src/traces/carpet/axis_attributes.js

@@ -36,6 +36,7 @@ module.exports = {
     title: {
         text: {
             valType: 'string',
+            dflt: '',
             role: 'info',
             editType: 'calc',
             description: [

src/traces/carpet/axis_defaults.js

@@ -203,6 +203,11 @@ module.exports = function handleAxisDefaults(containerIn, containerOut, options)
     // but no, we *actually* want to coerce this.
     coerce('tickmode');
 
+    if(!containerOut.title.text) {
+        delete containerOut.title.font;
+        delete containerOut.title.offset;
+    }
+
     return containerOut;
 };