Fix Tabular.from_xml_element for histogram case

[paulromano]

Description

Due to the change in #2981, I found that Tabular.from_xml_element didn't work for the case when len(x) == len(p) + 1. This PR fixes that.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

Thanks @paulromano! I like that the slicing is more explicit now. And thanks for addressing the XML parsing as well.