load_text only creates populations if none given

[hyanwong]

Fixes #1909

PR Checklist:

• Tests that fully cover new/changed functionality.
• Documentation including tutorial content if appropriate.
• Changelogs, if there are API changes.

[jeromekelleher]

Looks good. It would be nice to add a couple of minimal tests like (paraphrased)

def test_parse_text_no_populations(self):
    nodes = """\
       is_sample  time  population
        1                0      2
     """
     tables = parse_text(nodes)
     assert len(tables.nodes) == 1
     assert len(tables.populations) == 3

def test_parse_text_populations(self):
       populations = """\
       metadata
       metadata1
       metadata2
       """
       tables = parse_text(populations)     
       assert len(tables.populations) == 2
       assert tables.popualtions[0].metadata == "metadata1"
       assert tables.popualtions[0].metadata == "metadata2"

This will exercise both paths. I'm becoming a big fan of small obvious tests, they're so much more effective than the big elaborate code-based approaches that I used to favour.

[codecov]

Codecov Report

Merging #1910 (7f62573) into main (d467e0b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1910   +/-   ##
=======================================
  Coverage   93.16%   93.16%           
=======================================
  Files          27       27           
  Lines       25094    25094           
  Branches     1109     1109           
=======================================
  Hits        23379    23379           
  Misses       1681     1681           
  Partials       34       34           

Flag	Coverage Δ
c-tests	92.21% <ø> (ø)
lwt-tests	89.14% <ø> (ø)
python-c-tests	68.29% <50.00%> (ø)
python-tests	98.78% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
python/tskit/trees.py	97.79% <100.00%> (ø)

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[hyanwong]

Looks good. It would be nice to add a couple of minimal tests...

Good call. This exposed another bug: we were forgetting to strip the "\n" from each line when doing strict text reading. I've added this in, but apart from the test you suggested above, which only applied to the population table, this change doesn't have an associated unit test.

(I've also changed the strip("\n") to rstrip("\n") which is a bit more specific)

[jeromekelleher]

LGTM