fix: handle null and empty array in LinearSearch (issue #7340)

[Senrian]

Summary

Fixes issue #7340 - LinearSearch does not explicitly handle null or empty arrays, which can cause unexpected NullPointerExceptions or incorrect behavior.

Changes

• Added null check at the start of find() method
• Added empty array check (array.length == 0)
• Returns -1 for null/empty input, consistent with BinarySearch behavior
• Updated Javadoc to document the return value for null/empty cases

Behavior

Input	Previous Behavior	New Behavior
null array	NPE	Returns -1
Empty array []	Returns -1 (loop never runs)	Returns -1 (explicit)
Valid array, found	Returns index	Unchanged
Valid array, not found	Returns -1	Unchanged

This is consistent with how BinarySearch handles null/empty input.

Fixes #7340

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.46%. Comparing base (cc75b5e) to head (53c08b6).

Files with missing lines	Patch %	Lines
.../java/com/thealgorithms/searches/LinearSearch.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7347   +/-   ##
=========================================
  Coverage     79.45%   79.46%           
- Complexity     7083     7084    +1     
=========================================
  Files           790      790           
  Lines         23164    23166    +2     
  Branches       4556     4557    +1     
=========================================
+ Hits          18405    18408    +3     
+ Misses         4022     4021    -1     
  Partials        737      737           

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