Avoid panic after calling WordsSeq.

[trulede]

If the call to WordsSeq (mvdan.sh) returns an error, and that condition is not handled, then the resultant word with value nil is pushed to the list of words. A subsequent call to mvdan.sh will then panic.

This PR handles the error condition.

The panic.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x6286b7]

goroutine 1 [running]:
mvdan.cc/sh/v3/expand.FieldsSeq.func1(0xc0004a0930)
        /home/trule/go/pkg/mod/mvdan.cc/sh/v3@v3.12.1-0.20260124232039-e74afc18e65b/expand/expand.go:459 +0x97
mvdan.cc/sh/v3/expand.Fields(0xc00040e000, {0xc000482a80, 0x2, 0x2})
        /home/trule/go/pkg/mod/mvdan.cc/sh/v3@v3.12.1-0.20260124232039-e74afc18e65b/expand/expand.go:442 +0x125
github.com/go-task/task/v3/internal/execext.ExpandFields({0xc0000fe1e0?, 0x2d?})
        /home/trule/git/task/internal/execext/exec.go:139 +0x18e
github.com/go-task/task/v3/internal/fingerprint.glob({0xc00005a600?, 0x10?}, {0xc00005a570?, 0xc000110a20?})
        /home/trule/git/task/internal/fingerprint/glob.go:29 +0x37
github.com/go-task/task/v3/internal/fingerprint.Globs({0xc00005a600, 0x2d}, {0xc000482910, 0x2, 0x2e?})
        /home/trule/git/task/internal/fingerprint/glob.go:15 +0xea
github.com/go-task/task/v3/internal/fingerprint.(*ChecksumChecker).checksum(0x41d8a5?, 0x18?)
        /home/trule/git/task/internal/fingerprint/sources_checksum.go:91 +0x45
github.com/go-task/task/v3/internal/fingerprint.(*ChecksumChecker).Value(0xc0005924c0?, 0x0?)
        /home/trule/git/task/internal/fingerprint/sources_checksum.go:76 +0x13
github.com/go-task/task/v3.(*Executor).compiledTask(0xc0004623c0, 0xc0000cb240, 0x1)
        /home/trule/git/task/variables.go:201 +0x1014
github.com/go-task/task/v3.(*Executor).CompiledTask(...)
        /home/trule/git/task/variables.go:25
github.com/go-task/task/v3.(*Executor).RunTask(0xc0004623c0, {0x22b9750, 0x32f0ae0}, 0xc0000cb240)
        /home/trule/git/task/task.go:159 +0x256
github.com/go-task/task/v3.(*Executor).runCommand(0xc0004623c0, {0x22b9750, 0x32f0ae0}, 0xc00082afc8, 0xc0004c9820, 0x0?)
        /home/trule/git/task/task.go:382 +0x291
github.com/go-task/task/v3.(*Executor).RunTask.func1({0x22b9750, 0x32f0ae0})
        /home/trule/git/task/task.go:275 +0xa1e
github.com/go-task/task/v3.(*Executor).startExecution(0xc0004623c0, {0x22b9750, 0x32f0ae0}, 0xc00082afc8, 0xc000111b60)
        /home/trule/git/task/task.go:445 +0x9c
github.com/go-task/task/v3.(*Executor).RunTask(0xc0004623c0, {0x22b9750, 0x32f0ae0}, 0xc0004c9820)
        /home/trule/git/task/task.go:207 +0x5c5
github.com/go-task/task/v3.(*Executor).Run(0xc0004623c0, {0x22b9750, 0x32f0ae0}, {0xc0005f1d98, 0x1, 0x4})
        /home/trule/git/task/task.go:95 +0x40c

This also fix is also related to #2753