Fix search-form example in chapter7

[noobiept]

See if its an empty string.

[noobiept]

Yea that seems like a better way.

[suhailvs]

def search(request):
    query=request.GET.get('q', '')
    if query:
        message = 'You searched for: %r' % query
    else:
        message = 'You submitted an empty form.'
    return HttpResponse(message)