But it isn't a valid syllable in the orthography rules. Pinyin has a set of syllable onsets and a set of rimes. You determine the spelling of a syllable by choosing the onset/rime combination you want and looking it up in a table. Thus, the spelling of n-/-ü is "nü", and the spelling of ∅-/-ü is "yu". Same syllable column; zero characters in common in the spelling.

And if you look up ch-/-ing, the table will tell you "no such syllable", just like it will for "m-/-iang" or "h-/-weng".

All three of those are invalid in terms of orthography -- the standard specifically defines that there is no way to spell them. But "miang" is perfectly valid conceptually, and "ching" isn't even that.