make: pkg.mk: don't call git am if there are no patches

[kaspar030]

Contribution description

Don't call git am if there are no patches. Git should gracefully handle it, but somehow doesn't always (like, /dev/null broken).
And, no need to call git if a simple test is sufficient.

Testing procedure

Issues/PRs references

[kfessel]

The reason for that <dev/null is a non graceful handling of no file given (git am will try to read a patch from stdin)
The solution of this PR is check if git am should even be called (instead of piping /dev/null into stdin)

another possible source of nothing could be echo -n |

$(Q) echo -n | $(GIT_IN_PKG) $(GITFLAGS) am $(GITAMFLAGS) $(PKG_PATCHES)

not calling if not need seems to be the better solution

[kaspar030]

another possible source of nothing could be echo -n |

yeah, but that -n isn't posix?

even /dev/null was a fine solution. I had this fix done for the case where /dev/null was broken, before I understood that, to fix "empty patch" errors from git. I'd also be fine with just closing this. let's not spend more time here

[kfessel]

yeah, but that -n isn't posix?

posix is seems to be aware of -n https://www.unix.com/man-page/posix/1posix/echo/ but recommends using printf
so printf "" | would do

[kfessel]

murdock tested this and it is working as intended