build-pkgs from srpm fail when there is no tarball in srpm

Brief Description
-----------------
build-pkgs python-rpm-generators fail

Severity
--------
Minor

Steps to Reproduce
------------------
1. git apply patch attached (0001-add-python-rpm-generators-to-build-python3-rpm-gener.patch)
2. build-pkgs python-rpm-generators

Expected Behavior
------------------
The cmd run successfully

Actual Behavior
----------------
Here is the fail log:
"
01:59:23 b4: cat: /localdisk/loadbuild/sidney/starlingx/std/srpm_work/python-rpm-generators/extract_dir: No such file or directory
01:59:23 b4: === CANONICAL_SPEC_GIT=/localdisk/loadbuild/sidney/starlingx/std/srpm_work/python-rpm-generators/gits/python-rpm-generators.spec
01:59:23 b4: === TAR_DIR=python-rpm-generators-6
01:59:23 b4: === PATCH_TARGET_DIR=/localdisk/loadbuild/sidney/starlingx/std/srpm_work/python-rpm-generators/gits/python-rpm-generators.spec/python-rpm-generators-6
01:59:23 b4: === EXTRACT_TARGET_DIR=
01:59:23 b4: Something went wrong
01:59:23 b4: ERROR: srpm_extract_to_git (2491): tarball_extract failed while extracting '/localdisk/designer/sidney/starlingx/cgcs-root/cgcs-centos-repo/Source/python-rpm-generators-6-2.el7.src.rpm'
01:59:23 b4: ERROR: build_dir_srpm (747): failed to extract srpm '/localdisk/designer/sidney/starlingx/cgcs-root/cgcs-centos-repo/Source/python-rpm-generators-6-2.el7.src.rpm'
01:59:23 ERROR: reaper (1312): Failed to edit src.rpm from source at 'b4'
"

Reproducibility
---------------
100%

System Configuration
--------------------
N/A

Branch/Pull Time/Commit
-----------------------
latest code

Last Pass
---------
N/A

Timestamp/Logs
--------------
N/A

Test Activity
-------------
Developer Testing