First of all, what does "null-subject" mean? Taken from the Wikipedia page for "Null-subject languages":
[…] a null-subject language is a language whose grammar permits an independent clause to lack an explicit subject. Such a clause is then said to have a null subject.
Therefore, according to this, a non-null-subject language is a language that must have a subject. This can be expressed explicitly or through a pronoun and dummy or not1 doesn't matter.
Most Romance languages, for example when telling the weather (the sentence is "It rains") work without an explicit subject (1-5), but French is a (notable) exception and it doesn't allow sentences without expressing the subject (6), although in colloquial/oral usage, such sentences are allowed (7, 8):
- Piove. (Italian)
- Llueve. (Spanish)
- Plou. (Catalan)
- Chove. (Portuguese)
- Plouǎ. (Romanian)
- Il pleut. (French)
- (Je) m'en fiche. — (I) don't care.
- L'amour, (je) connais pas. — Love? (I) don't know.
Examples 7,8 are taken from Grevisse, Le Bon Usage, §234d.
I'm not aware of other Romance languages having the same behavior. I chose to treat the most relevant ones, but if you're aware of them, feel free to share them in your answer. Although I must say I'm more interested in French, if those have some deep relation to the evolution of the French language, then they might acquire importance.
I researched a bit before asking and I found an article titled "On Null subjects and related phenomena" by Asya Pereltsvaig, which treats about this phenomenon, comparing English and Italian. Apparently it is only slightly related to this question, but it does mention something that can be interesting:
[...] «Not only that, but historically, French used to be a well-behaved Romance language like Italian, Spanish, Romanian and Latin, allowing null-subjects (or missing subjects). What also becomes clear from the historical record is that French switched in all the relevant respects — at the same time!»
This article is hinting (well, more than hinting) to the fact that French used to work like other Romance languages, such as Spanish and Italian, but at a certain point in time, it changed.
If this is true, I can't prove it because I have nothing about the old phases of French evolution, so what I'm asking is: when and how did French switched and became a language that doesn't allow null-subject in independent clauses?
1: A dummy pronoun is used when there isn't an actual subject but it's nevertheless syntactically required by a given language. The usual example is "it rains": It's not that "something" rains, but English sentences can't stand without an expressed subject.