dispatch: gate against missing stdout/stderr

We do need procutil.stdin/stdout/stderr, but we don't care much for sys.std*.
Let's leave them be None as it is the Python 3 way.

mercurial/dispatch.py

@@ -166,6 +166,7 b' if pycompat.ispy3:'
         # "just work," here we change the sys.* streams to disable line ending
         # normalization, ensuring compatibility with our ui type.
 
+        if sys.stdout is not None:
         # write_through is new in Python 3.7.
         kwargs = {
             "newline": "\n",
@@ -174,9 +175,13 b' if pycompat.ispy3:'
         if util.safehasattr(sys.stdout, "write_through"):
             kwargs["write_through"] = sys.stdout.write_through
         sys.stdout = io.TextIOWrapper(
-            sys.stdout.buffer, sys.stdout.encoding, sys.stdout.errors, **kwargs
+                sys.stdout.buffer,
+                sys.stdout.encoding,
+                sys.stdout.errors,
+                **kwargs
         )
 
+        if sys.stderr is not None:
         kwargs = {
             "newline": "\n",
             "line_buffering": sys.stderr.line_buffering,
@@ -184,7 +189,10 b' if pycompat.ispy3:'
         if util.safehasattr(sys.stderr, "write_through"):
             kwargs["write_through"] = sys.stderr.write_through
         sys.stderr = io.TextIOWrapper(
-            sys.stderr.buffer, sys.stderr.encoding, sys.stderr.errors, **kwargs
+                sys.stderr.buffer,
+                sys.stderr.encoding,
+                sys.stderr.errors,
+                **kwargs
         )
 
         if sys.stdin is not None:
@@ -200,6 +208,8 b' if pycompat.ispy3:'
 
     def _silencestdio():
         for fp in (sys.stdout, sys.stderr):
+            if fp is None:
+                continue
             # Check if the file is okay
             try:
                 fp.flush()