botocore error on empty string for environment variables

[hmaarrfk]

conda-build/conda_build/environ.py

Line 416 in e7ad913

"REQUESTS_CA_BUNDLE": os.getenv("REQUESTS_CA_BUNDLE", ""),

is causing

https://github.com/boto/botocore/pull/3751/changes#diff-fe9f2a328fcc761f9d5cf26bf3e438716ffc27eb35da763a1f40ef8365662977R490

to trigger an error on environment that use boto core.

Could we change the syntax to simply not define REQUESTS_CA_BUNDLE in case it isn't present in the environment?