Formatting does not work if first line is blank

[karthiknadig]

Environment data

• VS Code version: 1.44.2
• Extension version (available under the Extensions sidebar): master
• OS and version: windows 10
• Python version (& distribution if applicable, e.g. Anaconda): 3.8
• Type of virtual environment used (N/A | venv | virtualenv | conda | ...): venv
• Relevant/affected Python packages and their versions: black
• Relevant/affected Python-related VS Code extensions and their versions: N/A
• Jedi or Language Server? (i.e. what is "python.jediEnabled" set to; more info How to update the language server to the latest stable version #3977): false
• Value of the python.languageServer setting: Microsoft

Expected behaviour

Should format document.

Actual behaviour

Formatting fails to apply.

Steps to reproduce:

1. Create a python file with blank first line:

import sys
print(sys.executable)

Reason

From the logs:

Formatting with black failed.
TypeError: Cannot read property 'length' of undefined

This issue is caused in getTextEditsInternal. When the first line is blank, start.line is 0. beforeLines[start.line - 1].length === 0 check fails with TypeError: Cannot read property 'length' of undefined