Fix override of position and check for content

If `position` is not supplied and target does not have the `[data-tooltip-position]` attribute, `@options.position` gets set to `null` preventing the default to override the value. This will check whether the data-attribute exists before attempting to override `position` otherwise keeping `@options.position` value `undefined` and letting the default position extend properly. 

Also in similar fashion, `content` will check for the `[data-tooltip]` attribute before trying to override the `content` and throw an error if there is no content to display. I can't imagine a situation where someone would want to show an empty tooltip...