Add printf null checks

[bynect]

Should fix #1305 . I added the null check to the places where a null string could occur. I may have missed some.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 34.61538% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 65.06%. Comparing base (764c382) to head (c70459c).

Files	Patch %	Lines
src/notification.c	0.00%	14 Missing :warning:
src/icon.c	33.33%	2 Missing :warning:
src/queues.c	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1323 +/- ## ======================================= Coverage 65.06% 65.06% ======================================= Files 48 48 Lines 8173 8173 ======================================= Hits 5318 5318 Misses 2855 2855 ``` | [Flag](https://app.codecov.io/gh/dunst-project/dunst/pull/1323/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/dunst-project/dunst/pull/1323/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | `65.06% <34.61%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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[bynect]

rebased