Fix: Return None in find_word_in_line when word is not found

[islml]

This PR addresses a TODO in the find_word_in_line function, which suggested that returning None when the word is not found would make more sense than returning a Range(-1, -1).

I modified find_word_in_line to return None when the word is not found as suggested, and updated find_word_in_code_line to handle the None return value

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.31%. Comparing base (f7c3ecd) to head (fc9a7f5).

Files with missing lines	Patch %	Lines
fortls/parsers/internal/parser.py	60.00%	2 Missing ⚠️
fortls/langserver.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #467      +/-   ##
==========================================
- Coverage   88.33%   88.31%   -0.03%     
==========================================
  Files          35       35              
  Lines        4800     4808       +8     
==========================================
+ Hits         4240     4246       +6     
- Misses        560      562       +2     

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[islml]

Changes:

1. Updated the validate_def function to handle the case where the Range is None.
2. Updated the _create_ref_link method to handle the case where find_word_in_code_line returns None.
3. Updated the build method in the Diagnostic class to handle the case where obj_range is `None.