Fixed missing checks for empty vertex arrays when updating the vertex buffers in sf::Text.

[binary1248]

Fixes #1558.

Test with the following in debug configuration (asserts enabled):

#include <SFML/Graphics.hpp>

int main()
{
    sf::RenderTexture foo;
    sf::Font bar;
    sf::Text baz(" ", bar);
    foo.draw(baz); // Debug assert 1 & 2
    baz.setFillColor(sf::Color::Green);  // Debug assert 3
    baz.setOutlineColor(sf::Color::Green);  // Debug assert 4
}