Fix failed assert when messaging non-contact

This fixes #35 (closed) by properly creating conversations when a number is entered into the text field on the conversation creation page.

@andresbs could you verify this fixes it for you?

Bug

In src/messagemodel.cpp, sendMessage gets the most recent message:

result = co_await sendMessageInternal(m_phoneNumberList.first(), text);

This fails if m_phoneNumberList is empty (and thus has no first element). The lack of phone numbers is ultimately caused in src/contents/ui/NewConversationPage.qml, which doesn't include the search field when creating a conversation. This fixes it to include the search as a phone number if no contacts are selected.