formatter_glue_or_sprintf: handling NULL or zero length message; addresses issue #74

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e88153b) 78.97% compared to head (2ba0bc2) 83.95%. Report is 71 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #75 +/- ## ========================================== + Coverage 78.97% 83.95% +4.97% ========================================== Files 10 10 Lines 509 561 +52 ========================================== + Hits 402 471 +69 + Misses 107 90 -17 ``` | [Files](https://app.codecov.io/gh/daroczig/logger/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Gergely+Dar%C3%B3czi) | Coverage Δ | | |---|---|---| | [R/formatters.R](https://app.codecov.io/gh/daroczig/logger/pull/75?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Gergely+Dar%C3%B3czi#diff-Ui9mb3JtYXR0ZXJzLlI=) | `100.00% <ø> (ø)` | | ... and [6 files with indirect coverage changes](https://app.codecov.io/gh/daroczig/logger/pull/75/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Gergely+Dar%C3%B3czi)

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