Fix negative X output value in MeasureTextEx

[mpv-enjoyer]

MeasureTextEx used to output negative X for empty strings with non-zero spacing, for example

#include <raylib.h>
#include <stdio.h>

int main()
{
	InitWindow(400, 600, "Raylib window");
	printf("%f \n", MeasureTextEx(GetFontDefault(), "", 10.0f, 0).x); // 0
	printf("%f \n", MeasureTextEx(GetFontDefault(), "", 10.0f, 1).x); // -1
	printf("%f \n", MeasureTextEx(GetFontDefault(), "", 10.0f, 2).x); // -2
	printf("%f \n", MeasureTextEx(GetFontDefault(), "", 10.0f, 15).x); // -15
	CloseWindow();
}

[raysan5]

@mpv-enjoyer thanks for finding this issue, I think it's better to move the check to beginning to avoid all processing in case of a string starting with '\0'. I'm changing it.