Both the basic agreement and rendition of account must be proved. “he rule that an account rendered and not objected to within a reasonable time is to be regarded as correct assumes that there was an original indebtedness, but there can be no liability on an account stated if no liability in fact exists, and the mere presentation of a claim, although not objected to, cannot of itself create liability. . . . In other words, an account stated cannot create original liability where none exists; it is merely a final determination of the amount of an existing debt.”