SETUPTOOLS_DSO_PLAT_NAME: equivalent treatment unset and empty

Helps with GHA, where conditional values are easier than
conditionally unset.

.github/workflows/build.yml

@@ -77,6 +77,7 @@ jobs:
         piparch: ["i686", "x86_64"]
         pyver: ["cp27-cp27m", "cp27-cp27mu", "cp36-cp36m", "cp37-cp37m", "cp38-cp38", "cp39-cp39"]
         exclude:
+          # manylinux2010 and above don't provide 2.7 in images
           - manylinux: "manylinux2010"
             piparch: "i686"
             pyver: "cp27-cp27m"

src/setuptools_dso/dsocmd.py

@@ -543,7 +543,9 @@ if _bdist_wheel:
 
             # So far PIP doesn't clear the environment for sandbox builds...
             # allow mangling of platform name.  eg. 'linux_x86_64' -> 'manylinux1_x86_64'
-            plat_name = os.environ.get('SETUPTOOLS_DSO_PLAT_NAME', plat_name)
+            new_plat_name = os.environ.get('SETUPTOOLS_DSO_PLAT_NAME', '')
+            if new_plat_name:
+                plat_name = new_plat_name
 
             return (impl, abi_tag, plat_name)