Allow inserting vectors of arbitrary size on an empty index and pick up index dimension on first insert

[Ngalstyan4]

It can be confusing when one creates an index on an empty table and they have specified real[] array dimensionality in a CREATE TABLE statement (see #197, thanks @min-mwei for point out!)

To make for better experience, we could delay the actual index creation until the first insert, when index creation happens on an empty table.