Add period to pop empty color page stack warning

[Sam Carter]

Dear TL list,

Given that this appears to be an engine specific feature, I was advised to ask here.

Consider the following example I borrowed from https://tex.stackexchange.com/a/583096/36296 :

```
\documentclass{article}
\usepackage{color}
\begin{document}
\setbox0\hbox{\color{red}hello}
\box0 world
\end{document}
```

If compiled with pdflatex, this produces the following warning:

```
pdfTeX warning: pdflatex: pop empty color page stack 0
```

The fact that -- unlike other warnings -- this one does not end with a period, confuses some log parsing tools like the log summary from TeXstudio. Is there any chance a period could be added to the end of the warning?

Cheers,
S.