hygiene: fix mutable default argument in vttests/common.py::sgr_n (#19028)

sgr_n previously used a mutable list (`seq=[]`) as its default parameter.
In Python, default arguments are instantiated once at
function‐definition time and then shared across all calls.
If any caller mutated that list (e.g., `seq.append(...)`), later
invocations would inherit the mutated state, producing unpredictable or
corrupted VT-test sequences.

src/tools/vttests/common.py

@@ -59,7 +59,18 @@ def clear_all():
 def sgr(code=0):
     csi('{}m'.format(code))
 
-def sgr_n(seq=[]):
+def sgr_n(seq=None):
+    """Apply multiple SGR (Select Graphic Rendition) parameters.
+
+    Parameters
+    ----------
+    seq : Iterable of int | None
+        Sequence of numeric SGR codes to emit. If ``None`` (default) an empty
+        sequence is assumed. A mutable default argument must be avoided to
+        prevent unwanted state sharing between calls.
+    """
+    if seq is None:
+        seq = []
     csi('{}m'.format(';'.join(str(code) for code in seq)))
 
 def tbc():