Math issues

[raffaeler]

I found some issues in math rendering:

1. Inline math (using $ ... $)
   The following expression is not rendered because of the final whitespace before the closing $:

$\sum $

This other is correctly rendered:

$\sum$

2. Default array alignment
   The following is not rendered because the l of the left alignment is missing.
   In Typora (which uses Katex and Pandoc as well) the \begin{array}{} is assumed to be "left" by default. There is no reason for Typora to mangle the Katex markup. If you are pre-processing the Katex markup for some reasons, there may be a bug there, otherwise it can be the online rendering service (but I am not subscribed to their services therefore I can't open an issue).

\begin{array}{}
1 & 1 \\ 0 & 0
\end{array}

This other is correctly rendered:

\begin{array}{l}
1 & 1 \\ 0 & 0
\end{array}

[raffaeler]

Another very strange use-case that renders in other viewers.

This is not rendered:

$$
\sum_k^\phantom{}
$$

This is rendered:

$$
\sum_k^{\phantom{}}
$$

The braces are not required for a single element as shown by this other example which is rendered correctly:

$$
\sum_k^\alpha
$$