fix(scripts): initialize reserved fields of lv_image_dsc_t and lv_image_header_t to prevent compiler warnings

[sparkles43]

Previously the LVGLImage.py script did not initialize reserved fields of the lv_image_dsc_t and lv_image_header_t. This led to a flood of compiler warnings about uninitialized variables.

This PR sets these variables to zero and NULL respectively.

[uLipe]

Thanks for the correction, this source of warning could be a pain on projects that promotes warnings to errors.

[lvgl-bot]

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[liamHowatt]

Just had a thought. If we replace these reserved fields someday, they will actually have to be anonymous unions so that old code assigning a value to reserved will still compile.

typedef struct {
    lv_image_header_t header;   /**< A header describing the basics of the image*/
    uint32_t data_size;         /**< Size of the image in bytes*/
    const uint8_t * data;       /**< Pointer to the data of the image*/
    union {const void * reserved; void * some_new_field};
} lv_image_dsc_t;

[XuNeo]

I think it would be automatically initialized to zero. https://en.cppreference.com/w/c/language/struct_initialization

If the compiler you used generates warning, then we can do the simple fix.
So looks good to me.