Handle importing whitespace-only seed phrases

Fixes MetaMask#6694

This changeset fixes our parsing of seed phrases during import to handle the
case where a user tries to import a seed phrase that consists solely of whitespace.
We no longer produce an error and instead treat it as an incorrect seed phrase.

ui/app/pages/first-time-flow/create-password/import-with-seed-phrase/import-with-seed-phrase.component.js

@@ -30,10 +30,12 @@ export default class ImportWithSeedPhrase extends PureComponent {
   }
 
   parseSeedPhrase = (seedPhrase) => {
-    return seedPhrase
-      .trim()
-      .match(/\w+/g)
-      .join(' ')
+    const trimmed = seedPhrase.trim()
+    if (!trimmed) {
+      return ''
+    }
+
+    return trimmed.match(/\w+/g).join(' ')
   }
 
   componentWillMount () {

ui/app/pages/first-time-flow/create-password/import-with-seed-phrase/tests/import-with-seed-phrase.component.test.js

@@ -43,5 +43,15 @@ describe('ImportWithSeedPhrase Component', () => {
 
       assert.deepEqual(parseSeedPhrase('  foo   bar   baz  '), 'foo bar baz')
     })
+
+    it('should return an empty string when given a whitespace-only string', () => {
+      const root = shallowRender({
+        onSubmit: sinon.spy(),
+      })
+
+      const {parseSeedPhrase} = root.instance()
+
+      assert.deepEqual(parseSeedPhrase('   '), '')
+    })
   })
 })