net/netstat: add nil checks to Windows OSMetadata implementation

The API documentation does claim to output empty strings under certain
conditions, but we're sometimes seeing nil pointers in the wild, not empty
strings.

Fixes https://github.com/tailscale/corp/issues/8878

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

net/netstat/netstat_windows.go

@@ -224,6 +224,7 @@ type moduleInfoConstraint interface {
 	_MIB_TCPROW_OWNER_MODULE | _MIB_TCP6ROW_OWNER_MODULE
 }
 
+// moduleInfo may return "", nil indicating a successful call but with empty data
 func moduleInfo[entryType moduleInfoConstraint](entry *entryType, proc *windows.LazyProc) (string, error) {
 	var buf []byte
 	var desiredLen uint32
@@ -249,6 +250,12 @@ func moduleInfo[entryType moduleInfoConstraint](entry *entryType, proc *windows.
 	}
 
 	basicInfo := (*_TCPIP_OWNER_MODULE_BASIC_INFO)(addr)
+	// GetOwnerModuleFromTcp*Entry is apparently using nil as an empty result
+	// under certain circumstances, so we check all the things.
+	if basicInfo == nil || basicInfo.moduleName == nil {
+		return "", nil
+	}
+
 	return windows.UTF16PtrToString(basicInfo.moduleName), nil
 }