Fix a Makefile conditional to test for empty string instead of 'none'

[bradcray]

I was hitting an issue where, due to not having CHPL_HOME set, CHPL_CXX was being set to the empty string and the 'make compile-arrow-cpp' step would fail as follows due to not having a C++ compiler specified (I'm still not sure where the '-' on the '-O3' option went...):

O3 -std=c++17 -c /Users/bradc/arkouda//src/ArrowFunctions.cpp -o /Users/bradc/arkouda//src/ArrowFunctions.o -I/Users/bradc/arkouda//dep/zeromq-install/include -L/Users/bradc/arkouda//dep/zeromq-install/lib -I/Users/bradc/arkouda//dep/hdf5-install/include -L/Users/bradc/arkouda//dep/hdf5-install/lib -I/Users/bradc/arkouda//dep/arrow-install/include -L/Users/bradc/arkouda//dep/arrow-install/lib -I/Users/bradc/arkouda//dep/arrow-install/include -L/Users/bradc/arkouda//dep/arrow-install/lib -I/opt/homebrew/opt/libiconv/include -L/opt/homebrew/opt/libiconv/lib -fsanitize=
make: O3: No such file or directory

The conditional that was designed to guard against this is the following:

ifeq ($(CHPL_CXX),none)
CHPL_CXX=$(CXX)
endif

but this didn't fire because CHPL_CXX wasn't unset, it was simply set to the empty string. Here, I've changed the logic to:

ifeq ($(CHPL_CXX),)
CHPL_CXX=$(CXX)
endif

to test for the empty string, which causes CXX to be set to g++ as intended.

[bradcray]

In putting this together, I found myself wondering whether the Makefile should be setting CHPL_HOME if it's not set by running chpl --print-chpl-home (since chpl is definitely required to build Arkouda). My thinking was that this would ensure that the CHPL_CXX was a version that was consistent with what Chapel would use.

But that was a bigger change than I wanted to take on, and that flag was only introduced in Chapel 1.31, so 1.30 users would face issues without some additional cleverness...

[bmcdonald3]

I've opened #2842 so we don't drop that. I believe Brandon is planning to drop our 1.30 support in the coming weeks, now that we no longer technically need to be supporting it, so can likely implement that soon and don't think it's worth the extra effort to move forward with that now, since we are going to be dropping 1.30 support so soon.