Don't warn when stringifying undef phrases (as ->parse generates)

This fixes a regression in 1.899 caused by the addition of `use warnings` in 5bcb255.  Namely, the following did not warn under 1.898, but does under 1.899:

```
perl -MEmail::Address -wle 'my ($e) = Email::Address->parse(q|foo@example.com (Comment)|); print "Got |$e|"'
```

Specifically, 1.899 generates:
```
Use of uninitialized value $phrase in pattern match (m//) at lib/Email/Address.pm line 427.
Use of uninitialized value $phrase in substitution (s///) at lib/Email/Address.pm line 429.
Use of uninitialized value $phrase in substitution (s///) at lib/Email/Address.pm line 430.
Use of uninitialized value $phrase in concatenation (.) or string at lib/Email/Address.pm line 432.
Got |"" <foo@example.com> (Comment)|
```