AutotoolsToolchain: empty None values from self fields + Refactor

conan/tools/gnu/autotoolstoolchain.py

@@ -152,14 +152,19 @@ def environment(self):
                             + apple_flags + extra_flags["ldflags"])
         self.defines.extend([self.ndebug, self.gcc_cxx11_abi] + extra_flags["defines"])
 
+        self.cxxflags = self._filter_list_empty_fields(self.cxxflags)
+        self.cflags = self._filter_list_empty_fields(self.cflags)
+        self.ldflags = self._filter_list_empty_fields(self.ldflags)
+        self.defines = self._filter_list_empty_fields(self.defines)
+
         if is_msvc(self._conanfile):
             env.define("CXX", "cl")
             env.define("CC", "cl")
 
-        env.append("CPPFLAGS", ["-D{}".format(d) for d in self._filter_list_empty_fields(self.defines)])
-        env.append("CXXFLAGS", self._filter_list_empty_fields(self.cxxflags))
-        env.append("CFLAGS", self._filter_list_empty_fields(self.cflags))
-        env.append("LDFLAGS", self._filter_list_empty_fields(self.ldflags))
+        env.append("CPPFLAGS", ["-D{}".format(d) for d in self.defines])
+        env.append("CXXFLAGS", self.cxxflags)
+        env.append("CFLAGS", self.cflags)
+        env.append("LDFLAGS", self.ldflags)
         env.prepend_path("PKG_CONFIG_PATH", self._conanfile.generators_folder)
 
         return env

conans/test/integration/toolchains/gnu/test_autotoolstoolchain.py

@@ -42,3 +42,29 @@ def test_extra_flags_via_conf():
         assert 'export CXXFLAGS="$CXXFLAGS -O3 -s --flag1 --flag2"' in toolchain
         assert 'export CFLAGS="$CFLAGS -O3 -s --flag3 --flag4"' in toolchain
         assert 'export LDFLAGS="$LDFLAGS --flag5 --flag6"' in toolchain
+
+
+def test_not_none_values():
+
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.gnu import AutotoolsToolchain
+
+        class Foo(ConanFile):
+            name = "foo"
+            version = "1.0"
+
+            def generate(self):
+                tc = AutotoolsToolchain(self)
+                tc.environment()
+                assert None not in tc.defines
+                assert None not in tc.cxxflags
+                assert None not in tc.cflags
+                assert None not in tc.ldflags
+
+    """)
+
+    client = TestClient()
+    client.save({"conanfile.py": conanfile})
+    client.run("install .")
+