grep: fix logic for empty patterns

[mknos]

• Checking length of $pattern is unreliable because it is valid for a pattern to have zero-length (meaning match-all)
• I found this when testing the -f option but the logic applies to other usages of grep

%md5sum a.c
c9fd7ff4cb5b01981a2a362f82f44c89  a.c
%perl grep -F '' a.c | md5sum 
c9fd7ff4cb5b01981a2a362f82f44c89  -
%printf "\n" > pat0 && perl grep -f pat0 a.c | md5sum
c9fd7ff4cb5b01981a2a362f82f44c89  -
%perl grep -e '' a.c | md5sum 
c9fd7ff4cb5b01981a2a362f82f44c89  -

[coveralls]

Pull Request Test Coverage Report for Build 11914073920

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 73.069%

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Totals
Change from base Build 11910111793:	0.0%
Covered Lines:	350
Relevant Lines:	479

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💛 - Coveralls