Fix handling empty values of NO_COLOR and FORCE_COLOR

Fix handling NO_COLOR and FORCE_COLOR environment variables to correctly
be ignored when they are set to an empty value, as defined
in the specification:

> Command-line software which adds ANSI color to its output by default
> should check for a NO_COLOR environment variable that, when present
> *and not an empty string* (regardless of its value), prevents
> the addition of ANSI color.

(emphasis mine, https://no-color.org/)

The same is true of FORCE_COLOR, https://force-color.org/.

doc/en/reference/reference.rst

@@ -1146,13 +1146,13 @@ When set to ``0``, pytest will not use color.
 
 .. envvar:: NO_COLOR
 
-When set (regardless of value), pytest will not use color in terminal output.
+When set to a non-empty string (regardless of value), pytest will not use color in terminal output.
 ``PY_COLORS`` takes precedence over ``NO_COLOR``, which takes precedence over ``FORCE_COLOR``.
 See `no-color.org <https://no-color.org/>`__ for other libraries supporting this community standard.
 
 .. envvar:: FORCE_COLOR
 
-When set (regardless of value), pytest will use color in terminal output.
+When set to a non-empty string (regardless of value), pytest will use color in terminal output.
 ``PY_COLORS`` and ``NO_COLOR`` take precedence over ``FORCE_COLOR``.
 
 Exceptions

src/_pytest/_io/terminalwriter.py

@@ -29,9 +29,9 @@ def should_do_markup(file: TextIO) -> bool:
         return True
     if os.environ.get("PY_COLORS") == "0":
         return False
-    if "NO_COLOR" in os.environ:
+    if os.environ.get("NO_COLOR"):
         return False
-    if "FORCE_COLOR" in os.environ:
+    if os.environ.get("FORCE_COLOR"):
         return True
     return (
         hasattr(file, "isatty") and file.isatty() and os.environ.get("TERM") != "dumb"

testing/io/test_terminalwriter.py

@@ -213,6 +213,12 @@ def test_should_not_do_markup_NO_COLOR_and_FORCE_COLOR(
     assert_color_not_set()
 
 
+def test_empty_NO_COLOR_ignored(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.setitem(os.environ, "NO_COLOR", "")
+    monkeypatch.setitem(os.environ, "FORCE_COLOR", "1")
+    assert_color_set()
+
+
 class TestTerminalWriterLineWidth:
     def test_init(self) -> None:
         tw = terminalwriter.TerminalWriter()