CA-427358: Fix default value of readlines

[GeraldEV]

If readlines fails to open a file or read the contents, it will fail and return an empty string. However f.readlines (the exepcted output) produces a list of strings. This inconsistency causes some error handling code to treat the output as a populated list of strings instead of skipping over it (as the command failed).

Fix the issue by consistently returning the correct data type.

[github-actions]

Python3 coverage comment from https://github.com/marketplace/actions/pytest-coverage-comment

File	Stmts	Miss	Cover	Missing
xen-bugtool	1571	171	89%	89, 94–95, 416, 573, 744, 763, 828–829, 843, 863–864, 870–871, 876, 886, 907, 912–914, 917–918, 921–922, 964–966, 1107–1121, 1123–1132, 1199, 1325, 1327, 1361, 1379, 1382–1384, 1387–1391, 1394–1395, 1399–1401, 1536, 1564, 1579, 1587–1588, 1651–1653, 1655–1660, 1663–1667, 1671–1672, 1674, 1725–1726, 1728, 1730–1732, 1735–1736, 1738, 1740–1742, 1745–1746, 1748, 1750–1753, 1756–1766, 1769–1773, 1783, 1795, 1805, 1809, 1844–1846, 1960, 1999, 2059, 2102, 2105–2106, 2157, 2159–2162, 2165–2175, 2269–2271, 2274–2276, 2310–2312, 2352–2353, 2355–2356, 2368, 2386
tests
conftest.py	14	0	100%
tests/integration
conftest.py	23	0	100%
namespace_container.py	35	1	97%	114
test_system_load.py	18	0	100%
test_xenserver_config.py	15	0	100%
utils.py	65	8	87%	27, 57–60, 66, 86, 89
tests/unit
conftest.py	64	0	100%
test_cmd_output.py	29	0	100%
test_dir_list.py	11	0	100%
test_dump_xapi_procs.py	11	0	100%
test_dump_xapi_rrds.py	66	0	100%
test_file_output.py	20	0	100%
test_filter_xapi_clusterd_db.py	87	0	100%
test_fs_funcs.py	14	0	100%
test_load_plugins.py	23	0	100%
test_main.py	141	1	99%	250
test_output.py	77	0	100%
test_process_output.py	11	0	100%
test_snmp.py	15	0	100%
test_xapidb_filter.py	21	0	100%
TOTAL	2331	181	92%

Tests	Skipped	Failures	Errors	Time
44	0 💤	0 ❌	0 🔥	2.154s ⏱️

[MarkSymsCtx]

Not anything to do with this PR itself but it's pretty horrid to not have any whitespace between these method definitions.