Fix blank slate crash (#42)

* Default to empty string if username/pw not in env

This ensures we prompt for username and password if they are not present in
arguments or in envvars (os.getenv defaults to None, not "")

* Prevent KeyError if providing empty username/password

csv-export.py

@@ -35,7 +35,7 @@
 while logged_in != True:
 
     if username == "":
-        username = os.getenv("RH_USERNAME")
+        username = os.getenv("RH_USERNAME", "")
     if username == "":
         print("Robinhood username:", end=' ')
         try:
@@ -45,12 +45,12 @@
         username = input()
 
     if password == "":
-        password = os.getenv("RH_PASSWORD")
+        password = os.getenv("RH_PASSWORD", "")
     if password == "":
         password = getpass.getpass()
 
     logged_in = robinhood.login(username=username, password=password)
-    if logged_in != True and logged_in.get('non_field_errors') == None and logged_in['mfa_required'] == True:
+    if logged_in != True and logged_in.get('non_field_errors') == None and logged_in.get('mfa_required') == True:
 
         if mfa_code is None:
             mfa_code = os.getenv("RH_MFA")