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sed  -E '/^\s*\/\*/!bx ; /\*\/\s*$/!bx ; /\*\/[^;]+;.*\/\*/bx ; d;  :x'

If the comment line contains a (second) comment end followed by statement ; and eventually by another comment start then there is code inside so branch past the delete to x

/\*\/[^;]+;.*\/\*/bx

Testing for a valid code construct (????; followed by /*) means that this will reject malformed syntax such as

/* between two comments */ x = 0;  could be some malformed code */

To keep any text between comments that may have been 'meant to be code' then this section of the regex would be changed to look for at least one character before the next comment is opened

/\*\/.+\/\*/bx

Both of the above rely on the comments being well formed.

Since we found no valid code then delete

/* Comment 1 */
printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
            /* Comment 4 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0;  /*could be some code */
/* between two comments */ x = 0;  some malformed code */

printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0;  /*could be some code */

sed  -E '/^\s*\/\*/!bx ; /\*\/\s*$/!bx ; /\*\/\s*\S+.*\/\*/bx ; d;  :x' draft 

If the comment line contains a clsing comment marker followed by at least one non-whitespace character and then another comment start then there is code inside so branch past the delete to x

/\*\/\s*\S+.*\/\*/bx

Since we found no valid code then delete

/* Comment 1 */
printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
            /* Comment 4 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0;  /*some code */

printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0; /*some code */

sed  -E '/^\s*\/\*/!bx ; /\*\/\s*$/!bx ; /\*\/[^;]+.*\/\*/bx ; d;  :x'

/\*\/[^;]+.*\/\*/bx

Testing for a valid code construct (; followed by /*) means that this will reject malformed syntax such as

sed  -E '/^\s*\/\*/!bx ; /\*\/\s*$/!bx ; /\*\/[^;]+;.*\/\*/bx ; d;  :x'

/\*\/[^;]+;.*\/\*/bx

Testing for a valid code construct (????; followed by /*) means that this will reject malformed syntax such as

Both of the above rely on the comments being well formed.

Since we found no valid code then delete

To keep any text between comments that may have been 'meant to be code' then this section of the regex would be changed to look for at least one character before the next comment is opened

/\*\/.+\/\*/bx

Since we found no valid code then delete