Ensure passwords can be read from options files (#180)

Fixes #166. Wowza what an issue. After spending way too much time
googling various forms of "can't read password from mariadb options
file", I started poking around the capi.jl file, and reading anything
related to "options". I was trying to see if there was a way to have a
debug print of what options were actually read from an options file.
That's when I found the following in the docs for `mysql_real_connect`,
which I'm now very thankful that past me decided to copy/paste all the
api docs for every single api function.

```
For passwd, specify a value of NULL. (For the password, a value of the empty string in the mysql_real_connect() call cannot be overridden in an option file, because the empty string indicates explicitly that the MySQL account must have an empty password.)
```

So if you want a password to be read from an option file, you need to
pass NULL to the `mysql_real_connect` function; but we were always
passing an empty string! So it prevented reading any password from
option files. Anyway, the fix is just that: by default we'll pass
`nothing` as the password, which will pass NULL to the actual api call,
allowing passwords to be read from option files.

src/MySQL.jl

@@ -21,7 +21,7 @@ mutable struct Connection <: DBInterface.Connection
     db::String
     lastexecute::Any
 
-    function Connection(host::String, user::String, passwd::String, db::String, port::Integer, unix_socket::String; kw...)
+    function Connection(host::String, user::String, passwd::Union{String, Nothing}, db::String, port::Integer, unix_socket::String; kw...)
         mysql = API.init()
         API.setoption(mysql, API.MYSQL_PLUGIN_DIR, API.PLUGIN_DIR)
         API.setoption(mysql, API.MYSQL_SET_CHARSET_NAME, "utf8mb4")
@@ -285,7 +285,7 @@ Connect to a MySQL database with provided `host`, `user`, and `passwd` positiona
   * `read_default_group::Bool`: only the default option groups are read from specified option file(s)
   * `option_group::String`: it is interpreted as a custom option group, and that custom option group is read in addition to the default option groups.
 """
-DBInterface.connect(::Type{Connection}, host::String, user::String, passwd::String; db::String="", port::Integer=3306, unix_socket::String=API.MYSQL_DEFAULT_SOCKET, kw...) =
+DBInterface.connect(::Type{Connection}, host::String, user::String, passwd::Union{String, Nothing}=nothing; db::String="", port::Integer=3306, unix_socket::String=API.MYSQL_DEFAULT_SOCKET, kw...) =
     Connection(host, user, passwd, db, port, unix_socket; kw...)
 
 """

src/api/capi.jl

@@ -1129,8 +1129,8 @@ If no value is found in an option file for a parameter, its default value is use
 Return Values
 A MYSQL* connection handler if the connection was successful, NULL if the connection was unsuccessful. For a successful connection, the return value is the same as the value of the first parameter.
 """=#
-function connect(mysql::MYSQL, host::String, user::String, passwd::String, db::String, port::Integer, unix_socket::String, client_flag)
-    @checknull mysql mysql_real_connect(mysql.ptr, host, user, passwd, db, port, unix_socket, client_flag)
+function connect(mysql::MYSQL, host::String, user::String, passwd::Union{String, Nothing}, db::String, port::Integer, unix_socket::String, client_flag)
+    @checknull mysql mysql_real_connect(mysql.ptr, host, user, passwd === nothing ? Ptr{UInt8}(C_NULL) : passwd, db, port, unix_socket, client_flag)
     return mysql
 end
 

test/my.ini

@@ -3,4 +3,5 @@ host=127.0.0.1
 user=root
 port=3306
 connect_timeout=30
-report-data-truncation=true
+report-data-truncation=true
+password = ""

test/runtests.jl

@@ -4,11 +4,11 @@ conn = DBInterface.connect(MySQL.Connection, "127.0.0.1", "root", ""; port=3306)
 DBInterface.close!(conn)
 
 # https://github.com/JuliaDatabases/MySQL.jl/issues/170
-conn = DBInterface.connect(MySQL.Connection, "mysql://127.0.0.1", "root", ""; port=3306)
+conn = DBInterface.connect(MySQL.Connection, "mysql://127.0.0.1", "root"; port=3306)
 DBInterface.close!(conn)
 
 # load host/user + options from file
-conn = DBInterface.connect(MySQL.Connection, "", "", ""; option_file="my.ini")
+conn = DBInterface.connect(MySQL.Connection, "", ""; option_file=joinpath(dirname(pathof(MySQL)), "../test/", "my.ini"))
 @test isopen(conn)
 
 DBInterface.execute(conn, "DROP DATABASE if exists mysqltest")