[pkg] Don't initialize _packages to empty set

When initializing _packages, a distinction must be made between not
having any package and not being initialized at all.
Otherwise, if a package manager returns no package, the query command
will execute hundreds of times.

This can reproduce with 'flatpak' on RHEL when no package is returned:

  # flatpak list
  --> no output

  # strace -fttTvyy -s 128 -e execve -o sos.strace -- ./bin/sos report
  [...]
  Press ENTER to continue, or CTRL-C to quit.
  ^C

  # grep -c ' execve("/usr/sbin/flatpak"' sos.strace
  350

Signed-off-by: Renaud Métrich <[email protected]>

sos/policies/package_managers/__init__.py

@@ -54,7 +54,7 @@ class PackageManager():
     files = None
 
     def __init__(self, chroot=None, remote_exec=None):
-        self._packages = {}
+        self._packages = None
         self.files = []
         self.remote_exec = remote_exec
 
@@ -63,7 +63,7 @@ def __init__(self, chroot=None, remote_exec=None):
 
     @property
     def packages(self):
-        if not self._packages:
+        if self._packages is None:
             self._generate_pkg_list()
         return self._packages
 
@@ -181,6 +181,9 @@ def _generate_pkg_list(self):
                               'pkg_manager': 'package manager name'}}
 
         """
+        if self._packages is None:
+            self._packages = {}
+
         if self.query_command:
             cmd = self.query_command
             pkg_list = self.exec_cmd(cmd, timeout=30, chroot=self.chroot)
@@ -340,6 +343,9 @@ def all_files(self):
         return self.files
 
     def _generate_pkg_list(self):
+        if self._packages is None:
+            self._packages = {}
+
         self._packages.update(self.primary.packages)
         for pm in self.fallbacks:
             _pkgs = pm.packages