fix logic for env-vars unset vs empty

conan/tools/env/environment.py

@@ -82,6 +82,9 @@ def __init__(self, name, value=None, separator=" ", path=False):
         self._path = path
         self._sep = separator
 
+    def __bool__(self):
+        return bool(self._values)  # Empty means unset
+
     def dumps(self):
         result = []
         path = "(path)" if self._path else ""
@@ -467,7 +470,7 @@ def save_ps1(self, file_location, generate_deactivate=True):
                 result.append(
                     f'if ($env:{varname}) {{ $env:{_old_env_prefix(filename)}_{varname} = $env:{varname} }}'
                 )
-            if value:
+            if varvalues:
                 value = value.replace('"', '`"')  # escape quotes
                 result.append(f'$env:{varname}="{value}"')
             else:
@@ -497,7 +500,7 @@ def save_sh(self, file_location, generate_deactivate=True):
                     f'export {_old_env_prefix(filename)}_{varname}="${{{varname}}}"; '
                     f'fi;'
                 )
-            if value:
+            if varvalues:
                 result.append(f'export {varname}="{value}"')
             else:
                 result.append(f'unset {varname}')
@@ -617,6 +620,7 @@ def _ps1_deactivate_contents(deactivation_mode, values, filename):
         Pop-Location
     """)
 
+
 def _sh_deactivate_contents(deactivation_mode, values, filename):
     vars_list = " ".join(quote(v) for v in values.keys())
     if deactivation_mode == "function":

test/integration/environment/test_buildenv_profile.py

@@ -132,3 +132,20 @@ def generate(self):
     assert "WARN: dep ENV:Foo" in client.out
     assert "WARN: pkg ENV:Foo2" in client.out
     assert "WARN: None ENV:Var" in client.out
+
+
+def test_buildenv_error_unset():
+    # https://github.com/conan-io/conan/issues/19285#issuecomment-3569891282
+    c = TestClient()
+    profile = textwrap.dedent("""
+        [buildenv]
+        CLASSPATH=!
+        OTHERPATH=
+        """)
+    c.save({"conanfile.txt": "",
+            "profile": profile})
+
+    c.run("install . -pr=profile -s:a os=Linux")
+    env = c.load("conanbuildenv.sh")
+    assert "unset CLASSPATH" in env
+    assert 'export OTHERPATH=""' in env