Fix shift in tree entries when initialized with empty array

There was a problem when not all branches are created at the same time – in such a situation a shift between corresponding values is introduced, see minimal working example below.

I see two issues:

not sure what should be done about empty iterables of other types (like dict) – so far nothing
this is low-code fix, but a special case in _fill_branch() is effecively created: if value is empty list then branch is created but not filled. Maybe one should split it into _create_branch() and _fill_branch()?

import numpy as np
import pandas as pd
import uproot
from pprint import pprint
from pyjetty.mputils import treewriter

def fill(tw, dummy_value, n_elem):
    tw.fill_branch('single', dummy_value)
    tw.fill_branch('array', [dummy_value+i for i in range(n_elem)])
    tw.fill_tree()

tw = treewriter.RTreeWriter(name ='t', file_name='test.root')

### SUCCESS:
# fill(tw, 100, 5)
# fill(tw, 10, 0)
# fill(tw, -1, 3)

### FAIL:
fill(tw, 10, 0)     # 'array' init with empty list – branch is not created
fill(tw, 100, 5)  
fill(tw, -1, 3)

tw.write_and_close()

print('file saved, check results:')
froot = uproot.open("test.root")
t = froot.get("tt")
arr = t.arrays(library="np")
try:
    df = pd.DataFrame(arr)
    pprint(df)
except ValueError as e:
    print('Error catched: ', e)
    pprint(arr)
    for k,v in arr.items():
        print(k, 'N=', len(v))

"FAIL" produces such tree, where array [100,101,...] is assigned to value 10 in "single" instead od 100

root [2] tt->Scan("single:array")
***********************************************
*    Row   * Instance *    single *     array *
***********************************************
*        0 *        0 *        10 *       100 *
*        0 *        1 *           *       101 *
*        0 *        2 *           *       102 *
*        0 *        3 *           *       103 *
*        0 *        4 *           *       104 *
*        1 *        0 *       100 *        -1 *
*        1 *        1 *           *         0 *
*        1 *        2 *           *         1 *
*        2 *        0 *        -1 *        -1 *
*        2 *        1 *           *         0 *
*        2 *        2 *           *         1 *
***********************************************

matplo / pyjetty

Fix shift in tree entries when initialized with empty array #88