fix: Fix ValueError when passing evals_dataframe to spans.log()

[nluzio]

Summary

Fix ValueError when passing evals_dataframe to spans.log()

Problem

When calling spans.log() with both a dataframe and an evals_dataframe, the method
would fail with:
ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(),
a.item(), a.any() or a.all().

This occurred because the code was using if evals_df: to check if the eval
dataframe exists, but pandas DataFrames cannot be evaluated directly in a boolean
context.

Solution

Changed all boolean checks on evals_df from if evals_df: to if evals_df is not None: to properly check for the existence of the dataframe.

Changes

Fixed 4 instances in src/arize/spans/client.py:

• Line 146: Index reset check
• Line 186: Column removal check
• Line 202: Values validation check
• Line 227: DataFrame merge check

The code already used the correct pattern (is not None) in some places but was
inconsistent.