curl: support zero-length argument strings in config files

... like 'user-agent = ""'

Adjusted test 71 to verify.

tests/data/test71

@@ -31,6 +31,7 @@ HTTP and -F upload in config file
 -F name=daniel
 -F tool=curl
 -F file=@log/test71.txt
+user-agent = ""
 </stdin>
  <command>
 http://%HOSTIP:%HTTPPORT/we/want/71 -K -
@@ -46,11 +47,10 @@ bar
 # Verify data after the test has been "shot"
 <verify>
 <strip>
-^(User-Agent:|Content-Type: multipart/form-data;|------------).*
+^(Content-Type: multipart/form-data;|------------).*
 </strip>
 <protocol>
 POST /we/want/71 HTTP/1.1
-User-Agent: curl/7.10.4 (i686-pc-linux-gnu) libcurl/7.10.4 OpenSSL/0.9.7a ipv6 zlib/1.1.3
 Host: %HOSTIP:%HTTPPORT
 Accept: */*
 Content-Length: 408