For once, I'd like something in Construct to be actually as easy as it seems.
This one probably IS actually as easy as it seems
Which high score table did you find on the forums? I've seen at least one, but I don't recall it working the way you're describing it, with a submit button and all. (It doesn't matter really; this is mostly for my own curiosity)
In any case, you can check for the length of a string; something like len(EditBox.Text) might suit you well here (assuming you're using an EditBox), and simply not do anything with it if the length is equal to 0.
As for the submit button vs. enter key issue, there's nothing stopping you from using a MouseKeyboard object to redirect the enter key. By setting the value in your high score table to left(EditBox.Text, len(EditBox.Text) - 1) when you're doing this via the enter key, you should be able to strip out the extraneous newline that would be attached to it otherwise.
Arrays are just a collection of values, which are referenced by number. For a 1-dimensional array (an array in which only one of X, Y, or Z is greater than 1 - and this is almost always X for ease of use), think of it like a row of numbers. For example, let's say you had an array of dimensions 6,1,1. Somehow it has values assigned to it of (10, 1000, 45, 23, 9999, 2). You ask the array to return the value at position 3, and it returns "45". It's basically just counting through the list.
For a 2-dimensional array (one in which 2 of X,Y,Z are greater than 1 - almost always just X and Y) think of it like a grid, or a chess board, or something. You ask for a value at (4,5), and it counts 4 spaces over and 5 spaces down and tells you what number's sitting in that space.
A 3-dimensional array's just a stack of those grids or chess boards, and the Z value picks which board.
Arrays are especially convenient, because you can simply save their current set of values to a file any time you like. Or, you can load to the array from a file, as needed. For a high score table, I'd recommend saving immediately after getting the new score on the list, and loading the array whenever you load the game (make it a global object).
Does this answer everything?