db: Drop None and empty-string checking in arguments.
db: Drop None and empty-string checking in arguments.

psycopg2 treats None and "" the same as not-provided:

```
assert connect(user="zulip", dbname="zulip")
assert connect(user="zulip", dbname="zulip", host="")
assert connect(user="zulip", dbname="zulip", host=None)
with Raises("no password supplied"):
    connect(user="zulip", dbname="zulip", host="localhost")

assert connect(user="zulip", dbname="zulip", port="")
assert connect(user="zulip", dbname="zulip", port=None)
assert connect(user="zulip", dbname="zulip", port=5432)
with Raises("could not connect to server"):
    connect(user="zulip", dbname="zulip", port=5000)

assert connect(dbname="zulip", host="localhost", password="right-password")
with Raises("no password supplied"):
    connect(dbname="zulip", host="localhost", password="")
with Raises("no password supplied"):
    connect(dbname="zulip", host="localhost", password=None)
with Raises("password authentication failed"):
    connect(dbname="zulip", host="localhost", password="wrong")
```

Co-authored-by: Adam Birds <a4b17ee8ffd707e7fe6b66c3737fdd8fbd524cdd@adbwebdesigns.co.uk>

punchagan/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,punchagan/zulip,kou/zulip,zulip/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,andersk/zulip,rht/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip