allow replacement with capture groups even if not all are participating in the match

[krystiannowak]

Currently when trying to use string replacement like in:
https://regex101.com/r/UFsDMr/1
so having regular expression:

^(BRAVO-(GOLF)|(ALPHA|BRAVO|CHARLIE|DELTA|ECHO|FOXTROT)).*

and test string:

BRAVO-CHARLIE something

and replacement:

${2}${3}

the following runtime error is thrown:

panic: runtime error: unsafe.Slice: ptr is nil and len is not zero [recovered, repanicked]

goroutine 11 [running]:
testing.tRunner.func1.2({0x104462a20, 0x104492aa0})
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1872 +0x190
testing.tRunner.func1()
	/opt/homebrew/opt/go/libexec/src/testing/testing.go:1875 +0x31c
panic({0x104462a20?, 0x104492aa0?})
	/opt/homebrew/opt/go/libexec/src/runtime/panic.go:783 +0x120
go.elara.ws/pcre.ConvertGlob({0x1043f40b3, 0x7})
	/somemydir/pcre/glob.go:56 +0x1f0
go.elara.ws/pcre.CompileGlob({0x1043f40b3?, 0x11e1add08?})
	/somemydir/pcre/glob.go:66 +0x20
go.elara.ws/pcre_test.TestCompileGlob(0x14000186a80)

as capture group not participating in the match are still tried to be accessed, but they should be allowed and if their value is not set should not produce any replacement value (= they should be an empty string).

This PR fixes that incorrect behaviour, so that the above regular expression replacement results in expected:

BRAVO

string.

[krystiannowak]

Raised also https://gitea.elara.ws/Elara6331/pcre/issues/7 for that