Fix git metrics for languages that don't support entity parsing

[tonysyu]

Summary

Check that currentGraph is defined addGitMetricToFileNodes before attempting to update git metrics.

Languages that don't support entity parsing attempted to update the graph with git metrics before currentGraph was defined on initial rendering.

Issue

To reproduce the issue (before this change), you can run the following in this repo:

python3 emerge.py -c py-template.yaml

using the following py-template.yaml (default python config with git_directory and git-metrics added):

---
project_name: py-example-project
loglevel: info
analyses:
- analysis_name: py check
  source_directory: emerge
  git_directory: ./.git
  only_permit_languages:
  - py
  only_permit_file_extensions:
  - .py
  file_scan:
  - git_metrics
  - number_of_methods
  - source_lines_of_code
  - dependency_graph
  - louvain_modularity
  - fan_in_out
  - tfidf
  export:
  - directory: emerge-output
  - graphml
  - json
  - tabular_file
  - tabular_console_overall
  - d3

Opening the output html (i.e. emerge-output/html/emerge.html) won't display a graph due to the error described above.

[greenled]

Tested it with a Go repo :+1: Thanks!

[glato]

@tonysyu Thanks for this fix 🚀, @greenled thanks for testing 💪.