Fix panic on empty --json flag value (#5498)

## Why

Found during a full-repo review of the CLI. Passing an empty value to
`--json` (for example `databricks jobs create --json=`, or `--json
"$VAR"` where the shell variable is unset) crashed the CLI with an index
out of range panic instead of a usable error message.

## Changes

Before, an empty `--json` value panicked; now it fails with a clean
validation error. `JsonFlag.Set` in `libs/flags/json_flag.go` indexed
`v[0]` to check for the `@file` prefix without first checking the
length. It now rejects empty input up front, and cobra surfaces it as
`invalid argument "" for "--json" flag: expected inline JSON or
@path/to/file, got an empty string`.

## Test plan

- [x] New unit test `TestJsonFlagEmptyValue` covering the empty value
case (panics without the fix, passes with it)
- [x] `go test ./libs/flags` passes
- [x] `./task fmt-q`, `./task lint-q`, and `./task checks` pass

This pull request and its description were written by Isaac.

Author: simonfaltum

libs/flags/json_flag.go

@@ -2,6 +2,7 @@ package flags
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -23,6 +24,9 @@ func (j *JsonFlag) String() string {
 
 // TODO: Command.MarkFlagFilename()
 func (j *JsonFlag) Set(v string) error {
+	if v == "" {
+		return errors.New("expected inline JSON or @path/to/file, got an empty string")
+	}
 	// Load request from file if it starts with '@' (like curl).
 	if v[0] != '@' {
 		j.raw = []byte(v)

libs/flags/json_flag_test.go

@@ -24,6 +24,13 @@ func TestJsonFlagEmpty(t *testing.T) {
 	assert.Nil(t, request)
 }
 
+func TestJsonFlagEmptyValue(t *testing.T) {
+	var body JsonFlag
+
+	err := body.Set("")
+	assert.EqualError(t, err, "expected inline JSON or @path/to/file, got an empty string")
+}
+
 func TestJsonFlagInline(t *testing.T) {
 	var body JsonFlag