Drop empty pip packages from the list

This patch aims to remove empty records from venv_pip_packages list
to avoid pip error due to that.

This also brings _venv_pip_packages variable, which optimize process by
doing union and sort only once.

Change-Id: Ic94f5a00346e47c394bd2cefc1cfca4ed8c3bdef

tasks/python_venv_install.yml

@@ -68,7 +68,7 @@
   copy:
     dest: "{{ venv_install_destination_path }}/requirements.txt"
     content: |
-      {% for item in (venv_default_pip_packages | union(venv_pip_packages)) | sort %}
+      {% for item in _venv_pip_packages %}
       {{ item }}
       {% endfor %}
   register: _requirement_file
@@ -124,7 +124,7 @@
   block:
     - name: Install python packages into the venv
       pip:
-        name: "{{ (venv_default_pip_packages | union(venv_pip_packages)) | sort }}"
+        name: "{{ _venv_pip_packages }}"
         state: "{{ venv_pip_package_state }}"
         virtualenv: "{{ venv_install_destination_path }}"
         extra_args: >-

tasks/python_venv_wheel_build.yml

@@ -80,7 +80,7 @@
       copy:
         dest: "{{ venv_build_host_requirements_path }}/{{ venv_install_destination_path | basename }}-requirements.txt"
         content: |
-          {% for item in (venv_default_pip_packages | union(venv_pip_packages)) | sort %}
+          {% for item in _venv_pip_packages %}
           {{ item }}
           {% endfor %}
       register: _requirement_file

vars/main.yml

@@ -76,3 +76,5 @@ venv_build_targets: |-
   {%   set _ = targets.__setitem__(distro, target_item) %}
   {% endfor %}
   {{ targets }}
+
+_venv_pip_packages: "{{ (venv_default_pip_packages | union(venv_pip_packages)) | sort | select | list }}"